Valve's GDC Talk: Porting Source to Linux [pdf]
This isn't my area but, they appear to be saying that by using OpenGL instead of Microsoft's equivalent, not only do they get Linux support and a stepping stone to mobile support, they can also use more up-to-date code that Microsoft doesn't support on XP (highlighting China as a place where they have modern machines running the old operating system) and get a 20% performance improvement even when going through a translation layer.That's a very interesting result.