On the other hand I don't see why Linux has to pose serious competition to Windows on the desktop, it's NOT as if the Linux desktop can 'break'.
If this new game push by Valve fails to gain traction, the Linux desktop will continue to chug along and slooowly increase it's market share just as it already is, if Valve succeeds I don't think for a second that it will pose a serious threath to the Windows desktop, even Apple with all their billions at their disposal haven't been able to do so, however it will make it alot nicer for those of us using Linux and it will certainly get more people onto Linux due to lowering the barrier of entry, but nowhere near enough for Microsoft to start sweating.
Again that's just my personal cynical/realist view, I would dance a jig if I was wrong and Linux gained massive desktop market share, but I just don't see it, not unless Microsoft would effectively abandon the desktop outright to start chasing Apple in some 'pad' goosechase but not even Ballmer could be that stupid... right?
So, for those of us already using Linux on our desktop, and those on the fence considering a switch to Linux, what Valve is pursuing could indeed have huge impact on our desktop experience so it's certainly exciting times ahead. Meanwhile looking past the desktop, Linux future couldn't be brighter in my opinion so if we are strictly talking 'Linux', it's already a incredible success.