本家のForumからコピペだよ

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.

訳：　Direct3D レイヤー？　屈辱！！

In practice this means for OpenGL under Aeroglass:

OpenGL performance will be significantly reduced - perhaps as much as 50%

訳：　性能が50%だって！！！　マジかよ

OpenGL on Windows will be fixed at a vanilla version of OpenGL 1.4

訳：　１．４までだって！！?　えー

No extensions will be possible to expose future hardware innovations

訳：　拡張クソくらえ！！by MS

It would be technically straightforward to provide an OpenGL ICD
within the full Aeroglass experience without compromising the stability or the security of the operating system.
Layering OpenGL over Direct3D is a policy more than a technical decision.


後は、OpenGL orgのForumでさんざん議論されてるから、読んでみてね
ttp://www.opengl.org/discussion_boards/cgi_directory/ultimatebb.cgi?ubb=get_topic;f=12;t=000001