No, you only need Impulse to 'activate' a game, or update it. Once a game is installed, a non-transferable license file or some such is present on your machine. The game checks for that license, then plays without Impulse.
Essentially, once you install a game with Impulse, you can uninstall Impulse completely and still play the game forever if you like. But you'll need Impulse any time you want to update an app or game, and any time you want to install it again.
In short:
Steam: DRM is always running, constantly calls home. Great integrated communications, matchmaking, huge selection of games, huge capital backing. Requires frequent internet connection, app will continually call home even in offline mode, unless internet is completely disconnected. Sneaky.
Impulse: DRM required on install or update only (for now). Beta-quality online community, IRC-style integrated communications. Good-sized game catalog, a little pricey for being a smaller player. Client not required to play game. Service claims to be DRM free, which it isn't, which is a little sneaky, but is generally 'forgiven' for this, given the competition.
GOG.com: No DRM of any kind. Forum-based community only. Small, but growing selection of games. For now, fixed prices $6-$9 US.

we're on irc, twitter, facebook, and youtube. see you there.

Ha ha, I stand corrected. Touché!

Steam: DRM is always running, constantly calls home.
Only calls home when checking for an update, or when you first start a game while online. If it can't get online, it'll try to contact the authserver but that's about it. It's not doing anything nasty to your PC.

While I agree it isn't doing anything nasty, I also assert it isn't doing anything necessary. At least Impulse leaves it up to the user to decide when and how upgrades and authorization occurs, Steam does this all the time while collecting statistics. There's a totalitarian air to it. That said, I don't particularly mind, I own several great Steam games, and the community integration makes the trade off worth it, but if I had the choice, I'd be happy to do without it.

I grabbed that over Christmas, as well. Really, really fun game!
Actually, Impulse typically does a sale like that every weekend, with something marked way down for a few days.

Gog doesn't COMPLETELY stand alone. Various indie devs release their stuff dirt cheap and with no DRM. Gog formalizes and guarantees it, though.
You have to actually look into it on a case by case basis when it's indie. Sometimes a indie games comes out with DRM, but they remove it like in the second patch (way faster than commercial shops). Aquaria, for instance, removed the DRM in the second big patch. Noitu Love 2 I think was DRM free from the beginning.
(both awesome games btw. Noitu Love 2 especially is my personal recommendation atm, considering it's dirt cheap price of $10 and Treasure-like sidescrolling gameplay. I even like the mouse controls, which I'd never have thought I'd like in a 2D action sidescroller).

What's with the weird putting up of one of the Dawn of War expansions instead of the other expansions or even basic games? Is THQ just trying out Impulse? It's weird to buy just PART of a game ... and not the basic parts with all the beginning races playable, at that. Seems really weird.
I'm also curious if Impulse really keeps all DRM off machines except for their own. Because THQ stuff including DOW is crawling with DRM these days.
If I recall, from reading reclaimyourgame.com, Titan Quest had DRM too.

Impulse doesn't serve DRM-free games. Stardocks OWN games are "DRM-Free," but third parties can use whatever they want. Impulse/Stardock notes the presence of any third party DRM on the game's page. For third parties, Impulse amounts to nothing much more than a delivery system.

Uh oh. Thanks extremely much for the heads up. I'm downloading DOW: Dark Avatar as I type this, but if it's got DRM in the Impulse version, I will never install it.
But the thing is ... I have looked at a few pages and seen no DRM yes/no or even mention of DRM whatsoever. Is there something else by the "game page" you mean that I don't know? I mean, there I am on the page you can purchase from, describing the game, requirements, etc. ... and not one hint about whether there is DRM or not.

WHY?
Sorry for shouting, but you paid for it, right? Why not play it?
Trust me, I'm definitely in the "hate's DRM" camp and could go on all day about how they're a bunch of greedy evil scum... but at the END of the day, as much as we whine and wank and weep... virtually all of the time the DRM doesn't actually end up doing anything bad to your computer, despite the hysteria even I tap into sometimes to convince laypeople otherwise. The number of verifiable cases of actual damage done to data and hardware that aren't bonafide mistakes are shockingly small; the stories just get circulated alot (look up "moral panic"). And if it's a matter of control and personal pride, just crack the game.
Seriously, play the game you bought.
EDIT: Check, for instance, Sacred 2. It mentions SecuROM and the 2 activation limit. I don't know if they're consistent about informing us.

I find it hard to even imagine what to say to that. You call yourself an anti-DRM guy but would sacrifice your computer, as well as your supposed principles, to play a $3.99 game?
Holy hell, man,what's going on there?
Both my computer and my principles are worth a lot more than $3.99.
Re: looking stuff up, take a look at reclaimyourgame.com. Look at reviews of DRM-laden games on Amazon.com. They point out real problems experienced by real people, not "moral panic." No need to muddy the waters with that kinda junk when you have screenshots. DRM is not some fairy tale ogre by far.

Yeah but the number of things ascribed to being 'DRM broke my computer' issues are quite probably user error or damaged hardware.
I'm a bloody experienced IT person and never had a single issues that was verifiably connected to DRM. Most of the time any problem stemmed from poor programming trying to use a driver in a way it's not supposed to. The number of times I've had to reinstall shit for people because they have a stick of dodgy ram and "KNOW that their hardware is perfect, it must be that program I installed" is really very high.
The isues I have with DRM is the ethical 'customers are the enemy' mentality where I have to prove I'm not a pirate whereas a pirate isn't even noticed. It's treating people with base level disrespect and IT'S NOT EVEN WORKING.

I don't have to sacrifice my computer. It rarely happens and when it does a legitimate uproar can be heard quickly. Gaming a year behind helps.
When's the last time you, personally, have ever been unable to play a game you bought? The last time Safedisc or Starforce or SecuROM ever actually crippled your (your) optical drive? How many crashes can you unequivably with 100% fact and evidence-based certainty were due to buggy DRM and not general system instability?
Look up "rainbow parties." With they way everyone was talking about them, one would swear every underage girl they knew must've been taking part, right? That many people can't be wrong.
The simple fact is that any time anyone has a problem with anything, the easiest answer, in their mind, is the answer. Especially if it's someone elses fault, or the fault of some greedy evil bastard. God forbid their system is poorly configured!
Sometimes it IS the boogieman. But that doesn't vindicate faulty reasoning.
The biggest legitimate complaints I have with DRM annoying disk checks, phoning home, obfuscated behavior, non-disclosure, etc. You know, REAL things.
Ever notice that the massive grassroots complaints to ban Starforce didn't start until AFTER Chaos Theory, a game that took an unprecedented year to crack? Hmm...
I hate DRM, but I like to keep the dialogue grounded in facts or ideological discussions of fair use. I only resort to the rabble rousing when it's time to rouse a rabble.
Wow, haven't had backup in a long time.

Acknowledging that DRM is a real problem is about as far from rabble rousing as saying that eating too much can make you fat. Sure, you can provoke argument as much as you like the other way, but it's not productive or worthwhile.
Example: My brother goes to Asia a lot. He works like hell but doesn't really know people there well enough to hang out with them every darn night, especially since they all smoke and drink like hell over there and he does neither. So even though he's very social, he faces some boring nights alone after his ridiculously long days. He might like a computer game. But is trying one out worth the chance of losing a disk drive, or any other kind of problem, on his computer? Not by a very long shot indeed.
You guys may be willing to take your computer use casually and roll lots of dice and recommend others do too, but that's the more ideological position by far. Some people just need to rely on their computers.
Ascribing every problem with DRM to someone's "system instability" is not just really reaching, it's misdirecting. Since I don't know you guys, I won't say purposefully. But I will say, for my brother, me, and countless others, it's being done irresponsibly.