From the Wiki on Dx vs. OpenGL:
Windows Vista will include three OpenGL implementations. The first maps
OpenGL calls to Direct3D ones. This allows for hardware acceleration
through OpenGL from a standard install, but this implementation only
implements OpenGL 1.4, and features of versions after this must be
accessed as extensions. The second uses legacy Installable Client Drivers
(ICD) available for Windows XP. This will disable the Aero desktop but
otherwise functions as expected. The third is a full implementation that
will work alongside the desktop and still allow access to the GPU.