The continuing development of Vista has just taken a big leap forward; details have surfaced regarding the way OpenGL will behave on the upcoming OS, it was mentioned quite some time ago that Microsoft would simply layer the OpenGL API over the top of Direct3D, crippling performance quite severely as a result for all OpenGL based applications when using one of Vista's biggest features, the "Aero Glass" 3D desktop.
Turns out Microsoft have finally gotten some sense into them and will fully implement OpenGL so that it runs independantly of anything relative to DirectX.
More info on this over at the DailyTech folks.
The problem with OpenGL was that it was never officially supported or evangelized by Microsoft. Of course, Microsoft has always advocated its own Direct3D driver model over OpenGL, but GPU manufacturers have long supported OpenGL in drivers. Many games and productivity applications still use OpenGL even today. With Windows Vista, Microsoft made it clear that OpenGL support would only work as a layer sitting on top of Direct3D. There was going to be translation involved and thus, a performance hit.