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.