The real interesting part is that the AppleTV would surely be an ARM platform and Valve are exclusively an Intel platform. Given the price point it could easily be both.
I agree that being compatible with whatever hardware Valve is making would be pretty awesome.
Battery life isn't an issue in the home and ARM can't really do full fledged ports of the most demanding and popular AAA console titles. So I'd expect an Intel chip. It could possibly have an ARM chip too but I'd guess emulation is good enough and higher profit. Alternatively they could put in a bunch of ARM chips but iOS just makes a lot more sense on paper. With an Intel chip you gain the whole iOS library and the whole OS X library. At that point the library and capabilities may be good enough to forego other console purchases for many consumers.
Not to mention, the current Mac Minis run dead silent with fairly speedy x86 chips.