As a longtime PC gamer, any Microsoft store is roughly 0 threat to Steam from my perspective unless publishers stop publishing on Steam, which would seems very unlikely unless Microsoft took much less of a cut and Valve refused to follow suit, or Microsoft decided to pay publishers off for exclusives. Microsoft's past attempts to make Steam-like services have been completely laughable/extremely frustrating (see Games for Windows Live, their attempt at an online community/DRM solution).
I think they're expanding to Linux entirely because they want to expand to and support Linux, not because of any competitive pressure forcing them to retreat to Linux. A couple years ago they expanded to Mac, and this is just a natural extension.
IIRC , Windows 8 will only allow "Metro Style" apps to be installed via Microsoft's own store. I'm not sure whether a game would classify as "Metro Style" or not and since ARM based Windows tablets will be Metro only that kills the possibility of Steam to expand in that direction.
So there is the possibility that MS could turn around and say "No, Steam is banned from Windows" in the same way that apple would with the iPad.
This at least gives Valve the power to threaten MS with "FU, we're going to Linux and taking HL3 , Portal 3 and CS:GA with us"
I think they're expanding to Linux entirely because they want to expand to and support Linux, not because of any competitive pressure forcing them to retreat to Linux. A couple years ago they expanded to Mac, and this is just a natural extension.