I'd heard of this before, but never actually got linked to any discussion on it. I suppose that you can't get any more official than a thread on opengl.org, can ya? Since games like Doom, RtCW, & Quake (to name a few) use OpenGL, I can assure you that adoption of Windows Vista by any PC gamer's going to be done with much kicking and screaming...

"Microsoft's current plan for OpenGL on Windows Vista is to layer OpenGL over Direct3D in order to use OpenGL with a composited desktop to obtain the Aeroglass experience. If an OpenGL ICD is run - the desktop compositor will switch off - significantly degrading the user experience.
In practice this means for OpenGL under Aeroglass:

• OpenGL performance will be significantly reduced - perhaps as much as 50%
• OpenGL on Windows will be fixed at a vanilla version of OpenGL 1.4
• No extensions will be possible to expose future hardware innovations"