In general apps use whatever OpenGL system is available to them. That's up to the OS and graphics drivers to decide. As far as I know Vista *already* dropped software OGL, but it was so seldom used in XP anyway (and when it was used performance was horrible) that it's no big loss. People have made a big deal over nothing with that, really, with many people claiming Vista wouldn't "allow" OGL or would somehow have OGL calls going through DX10, making it slower. None of that is true.
As with XP it's up to the graphics card driver to provide a proper OGL-supporting driver, which does go through an abstraction layer in the driver itself just like DX does where it is translated into "graphics card speak" and then sent to the card for execution. Anyway I'm figuring the newness of Vista and driver support for it would be the reason for poor OGL stability/consistency. Not at all a surprise really.
- Oshyan