So, I can connect to Xbox live through my 360, I can download game demos and arcade trials, so obviously I'm online and thus connected to the network, using a D-link wireless/ethernet bridge that the xbox accepts and seems to treat as if it was the official wireless adapter (this one's model number, though it looks like that one). Yet when I try to use the windows media center connect thingy it says:
Your console isn't connected to your network. make sure your console is connected to your home network using an Ethernet cable or wireless connection.
I'd prefer not to pull a cable right through the room, and I don't have one handy anyway (strangely enough, I seem to have just about every other imaginable computer accessory and spare part), but I'm thinking that if online works the LAN functions should as well, no?

Because I want it that way - I get 5 public IP addresses, so I use one for the router, one for the server (I trust in the OS X firewall), one for the main box to not having to bother with port forwarding (and I trust in the Win7 firewall, no matter how stupid some people think that may be), and 2 are unused so far.
Funny, Windows can see the Xbox, but the Xbox won't see the network any further than than that it gets an IP address and all other IP settings from the router (DNS, gateway...).
The router has UPnP on, yes. Does the Xbox even have a setting for that?

I don't have a clue about anything Xbox related.
I did find and [url=http://news.softpedia.com/news/Troubleshoot-Your-Wireless-Networking-Adapter-Xbox-360-75602.shtml]this.
I don't really know if any of it helps with your problem...

Nah, those seem to deal with connecting to the wireless network and to Xbox Live, both of which already works (can download demos - that should be the ultimate test, right?), but when actually running the built-in tests - or trying to run the media center setup - it fails and claims to not get any sort of connection.

Its a shame you don't have a cat5 cable just so you can test that vs your wireless.

Heh, waddaya know. All of a sudden it works, t'was probably the magic of that music box in Fable 2 I just played for a little while.
Bonus points for all of you trying to help (ie. everyone so far), even if there was no real solution.
Er, wait, edit: No, Media library works, but Windows media center connect does not... ah, no matter, I can watch video from the computer on the xbox is really what I was looking for (though it IS strange).

If the Media Library works, and you can watch videos through it, it's probably using PS3 Media Server, if you haven't uninstalled it yet.

I'm finding both Windows' built-in sharing and the PMS one (which i don't think I have any real need of).

Nope, don't need PS3 MS unless you need to transcode the files in stream for the XBox to read them.

I already tried a number of movies and they played fine, just about everything today is some form of mpeg4, and when trying to play the first movie (a divx, I think, or maybe xvid), the xbox prompted me to download a media update allowing for mpeg4-based formats, subsequently installing in just a few seconds.
WIndows media should just play straight off, probably mpeg2 as well (as the console can play DVD, and thus contains an mpeg2 decoder), so the media support should last me quite a while.

Have you done the media centre connect setup? It spits out a code you have to enter on the PC to authorise the 360. If memory serves that would basically turn your 360 into a media centre extender which is pretty much a remote desktop like thing.
If you just want to access the files straight from your 360 without pissing about, the easiest way is to use media player 11, have everything in the library (am I the only one who hates these bloody programs having libraries?) and enable media sharing, it should deetect devices on your network and ask you to tick the ones you want to share to.
That said, if it works now and nothing is going wrong on the PC end, declare victory and move on

I can't, that's where this thread started. The Windows media player setup on the 360 refuses to accept that it's connected to a network, ditto the various connectivity tests the 360 can run.
Yeah, it's half a victory, I can at least browse the WiMP11 libraries and play the files (I have no problem with libraries, and rather like iTunes' one, not really the WiMP's though).