I'm using ATI Mobility Radeon X2300 with its proprietary fglrx driver. However, I reproduced the same bug in VirtualBox with the OpenSuSE KDE 4.1 livecd by following the above steps. So I think it's likely to be an upstream bug, other than a problem with the Ubuntu package or my video driver. The old KDE3 version of the game doesn't seem to have this problem.
I'm using ATI Mobility Radeon X2300 with its proprietary fglrx driver. However, I reproduced the same bug in VirtualBox with the OpenSuSE KDE 4.1 livecd by following the above steps. So I think it's likely to be an upstream bug, other than a problem with the Ubuntu package or my video driver. The old KDE3 version of the game doesn't seem to have this problem.