Microsoft's first technical beta of Vista layers OpenGL over Direct3D in order to use OpenGL with a composited desktop to obtain the Aeroglass experience. If an an application runs using a high-performance OpenGL ICD - the desktop compositor will switch off - significantly degrading the user experience. Write to your preferred software developer, hardware developer and video card manufacturer and tell them to make sure Microsoft solves this problem before release and fully supports OpenGL ICDs within Aero. Hardware and software vendors listen to developers. Don't be passive - send those emails and keep the topic in the foreground.
Source: http://www.opengl.org/

I've also read elsewhere that only OpenGL 1.4 will be supported which means no shaders and other fancy stuff will be available for OpenGL in Windows VISTA. Also, I've read that OpenGL performance will be reduced by something like 50%.

Although I don't particularily use OpenGL, I find it difficult to believe that OpenGL will really be layered over Direct3D. Wouldn't that make a significant hit on any OpenGL-based games? Can anyone provide further information on this topic?