They waited all these years to gain a significant foothold in the Windows segment. Now that they've got this, we could see them slowly expanding to Mac first, and today Linux. I mean, it's certainly a huge incentive if you're given ready access to your game library on the other OS, even though you didn't expect it when you purchased the game on Windows months or even years ago. Screams of extend. I just wait to see if the market forces themselves will play the 'extinguish' card or will Valve play it?
Once Office and a decent number of games port over to the penguin, I'm really confident that the only people who use Windows will be (a) people who know nothing about computers and use whatever it came preinstalled with (b) at work since they're not allowed to format the computer (c) they need a very specific software that is not available on Linux or Mac (Ansys etc.)
Realistically Steambox would seem to be the most obvious place for this to start happening. Of course how much the Steambox will be a "real" Linux computer vs just a kernel+whatever the minimal software stack is required to get Steam running remains to be seen.
I wouldn't hold out much hope for an Office for Linux regardless of what Phoronix thinks. Porting office to Linux would be a tantamount to MS admitting they have given up on Windows and would probably hurt their stock price significantly.
From a technical point of view the biggest thing that would make Linux a viable choice for the majority of desktops would be a WINE layer that provides at least as good compatibility as Windows XP for Windows software thus allowing legacy VB6 etc stuff to be moved over.
Or OpenGL graphics programmers who find the state of drivers under Linux, while a lot better than years ago, still sub-optimal compared to Windows...