Twitty, how about if you edit your mame.ini and change gl_vbo to 0 ?
See if that works. With APPLE YUV enabled, my Mac Pro with NVIDIA GeForce 8800 GT card didn't produce any OpenGL video at all for Cube Quest. With YUV disabled, it works fine both on my MacBook Pro ATI Mobility Radeon X1600 and the Mac Pro.
Not sure what's up with your iMac. Which OS are you running? I'm on 10.5.7. Have you done all the appropriate EFI, SMC and graphics firmware updates?