Do you guys think there will be a big Steam sale next month? What about in September?

Why would there be?

Are they not allowed to have a big sale in August?

The next "big" sale will be the winter one. They may run the occasional holiday sale in between, but they are never big or jaw dropping. This is pretty much their standard cycle.

They just had a big summer sale. The next one will probably be the holiday sale at the end of the year.

Isnt Quakecon sale supposed to be soon? That will include all id's games and bethseda games, should do anyway, That will be on sometime in august

And it's a good thing also. I don't have enough money for every month sales :)
Twice a year is plenty.

Pretty much this. Steam currently hold three main sales. The summer sale, pre-holiday sale (around Thanksgiving, ie., November), and the holiday sales.

If there are any other large sales, then those will be new this year.

However, GamersGate are going to have a large summer sale soon...

... GamersGate hasn't started their summer sale yet? Wow. I know Direct2Drive is, or was, anyway.

I hope we'll get some Halloween discounts on scary games, at least.

Not yet. The only discount I know which will be in that sale at some point is for Red Faction Armageddon. Which'll be nice, as I'm wanting to buy that and Valve won't sell it to me.

Is that a Steam sale?

Yup, They done it last year and the year before that so i dont see why not this year, Its not a Massive sale just bethesda and id games, still a good sale usually