It is because you have to use Mesa, which might be running iin software.
There is an acceleration package for the many graphics card, but I don't know anything about it.
nVidia is being very supportive of Linux: their TNT2 and GeForce cards work under Linux with hardware-acceleration of openGL. (but the drivers are proprietary)
radeon cards should also be supported very well under linux (even with open-source drivers)