It seems like Microsoft has really tied Valve's hands here. Unless something has changed recently, all apps installed on Windows RT will have to come from MS's app store (marketplace? bazaar?), which doesn't bode well for the future of Steam on the platform. I'm hoping they end up doing a lot more than porting Steam to Linux, because outside of Valve's efforts, most of the games on there are not (and will likely never be) Linux compatible.
This would mean that (pretty much) the only way you could game on non-Apple hardware would be to install Linux, which would make Linux the go-to OS for gaming, something I doubt Microsoft would let happen.
I really doubt Microsoft wouldn't allow Steam to run on Windows.
