I ran nexuiz-linux-glx.sh, and the game ran. However, the graphics were flashing between black and the nexuiz main menu at a high speed. I pressed F10 to close Nexuiz 2.5.1, then my desktop appeared, and everything on the screen graphicly scrambled beyond recognition. At that point, it was obvious that Ubuntu stopped responding because my mouse cursor would not move as I moved the mouse (evidenced by the fact that I could still see pieces of it when the screen scrambled); and I could not drop to a text console.
I was using the vesa graphics driver on Ubuntu 9.04 32bit desktop, with an integrated ATI x1270 graphics controller. I cannot get official graphics drivers for Ubuntu 9.04 because ATI dropped support for my graphics chip, making Ubuntu 8.10 the last Ubuntu version they released drivers for.
I'll play Nexuiz on Windows until this gets sorted out.