DailyTech writes OpenGL will be natively supported in Windows Vista without layering over Direct3D.
Direct3D. Using standard Windows installable client driver (ICD), OpenGL will be fully accelerated and be fully compatible with Windows Vista's Aeroglass UI. In fact, Khronos says that by the time Windows Vista ships, Aeroglass performance on OpenGL will be superior to that of Direct3D. According to Khronos and NVIDIA:
However, the OpenGL ICD drivers must still be downloaded and will not ship on the Windows Vista installation disc. Khronos said that NVIDIA already has a beta 2 ICD OpenGL driver available and ATI will release its own soon. If no ICD is present, Windows Vista will rely on the layered OpenGL mode by default and only offer basic functionality.