Truly, truly, epic news!
[edit] I wouldn't mind having a distro made by steam, but honestly, I doubt it. If they made their own console, and it did have an embedded Linux OS, I don't think they would bother trying to "port" it to PC. The benefit just doesn't seem to be there. As for why they are moving into Linux, I think its due to Windows 8. I know I started using Linux as a direct result of Vista. XP was showing its age and the alternative for me, since my computer is custom made, was Linux (I eventually ended up with Ubuntu by the way, and am now using Mint). I think people will feel like that this time around as well with W8 looking like well dressed garbage.