From the OMG! Ubuntu! blog: "Mass-Market Games Engine 'Unity 3D' to Support Linux": "The viability of Linux as a gaming platform continues to grow, with today’s news that Unity 3D (no, not that one), one of the most popular games engines used by developers of all calibres, will be supporting Linux."
Why the switch? Google "unity 3d linux" and you'll find a bunch of links to Unity forum threads asking for Linux support, typically with people responding that there aren't enough Linux users to matter, etc. What changed their minds?
Well... Windows 8 isn't getting good reviews (google "windows 8 unmitigated disaster", or, more to the point, check out "[head of Valve] Gabe Newell Wants to Support Linux Because Windows 8 is a 'Catastrophe'"), but there's more to it than that. With Windows 8, Microsoft is trying to make itself into Apple. It's putting out hardware (computers, not just mice and keyboards), and hence OEMs are worried. Most of all, it's moving towards an App Store as the way people get applications for their Windows computers, giving Microsoft control and making applications developers... in particular for this discussion, independent game developers, worry. (See "Why is Windows 8 'a catastrophe for anyone in the PC space'?" for more info.)
I am hoping this is the "The more you tighten your grip, Tarkin, the more star systems will slip through your fingers" scenario. I hope that the Unity people will work with (lean on? :)) graphics hardware makers as Valve has, making it even clearer that it's to their benefit to seriously support Linux. Things are, I hope, about to get interesting.
P.S. For non-Ubuntu users, the "no, not that one" aside above is there because Canonical, the makers (aggregators?) of Ubuntu, named their "let's impose a touch UI on everybody" UI "Unity", and it's had, at least until very recently, 2D and 3D versions.