If the extension or the version of OpenGL you''re looking for is present, then every feature for that extension or version is available. If the hardware itself does not support it, the driver have to do it in software instead. So if ARB_vertex_program is exposed as an extension for the TNT2 you mention, you can use it.

Even DirectX does not emulate everything. Most prominent example are the pixel shaders.
And I don't think that Direct3D can emulate texture units that are not available in HW.

[edited by - VolkerG on January 26, 2004 7:04:56 AM]