With the release of KDE 4.3, I added the PPA archive for Jaunty and upgraded from the previous release.
Now, the computer boots to the log in screen just fine, but logging in causes the computer to instantaneously reboot the machine.
Creating a brand new user account and logging in also suffers the same problem.
Update: Changing the video driver from radeon to vesa stopped the system from instantaneously rebooting, however the machine freezes at the KDE loading screen (even magic keys don’t work!). Looks like something to do with hardware initialisation, perhaps kwin trying to enable something fancy.. not sure.
Update 2: I was able to log into KDE with the vesa driver and disable desktop effects. After a reboot the system no longer crashes with the radeon driver. I then re-enabled the desktop effects and rebooted. This time I can log in, the effects are enabled and it doesn’t crash. We’ll see if it lasts!