Hi Theo,
Reading this
bug report, it seems as if this is a known issue with certain hardware platforms on particular releases of Ubuntu. For the Ubuntunites, it seems to come down to buggy drivers and/or a failure to load the necessary modules for 3d rendering. I don't know what disro you're using, so I don't know how much help that is for you however I'd check the Xorg log and see if you're getting an error similar to
(EE) fglrx(0): atiddxDriScreenInit failed, GPS not been initialized.
(WW) fglrx(0): ***********************************************
(WW) fglrx(0): * DRI initialization failed! *
(WW) fglrx(0): * (maybe driver kernel module missing or bad) *
(WW) fglrx(0): * 2D acceleraton available (MMIO) *
(WW) fglrx(0): * no 3D acceleration available *
(WW) fglrx(0): ********************************************* *
They suggest several possible solutions.