After installing the 10.5.1 update, my computer would not boot, and I got some weird error message about the CPU. I'm wondering if it's related to the Sonnett dual 1.8 GHz processor upgrade in my G4 (AGP graphics.) I tried to repair using the original Leopard disk - didn't work. I then had to do a complete restore (erasing my hard drive) from Time Machine (which took 2 hours.) I was also unable to get into single-user mode. I then unplugged all USB and Firewire connections and tried installing the 10.5.1 update again. This time there was no kernel panic after reboot, but it hung at the grey Apple screen. I am again restoring to 10.5.0, and have no plans to try this update again.