I doubt many people will experience the same thing, but I might be able to save somebody some trouble in tracking it down. There's a small kext called DoubleCommand that I applied quite some time ago that converts the enter key to the right of the space bar on powerbooks into a second command-key. Unfortunately, it does not play nicely with 10.1.5.
This can be fixed easily by booting from a CD (OS 9 is fine), use Sherlock to find the DoubleCommand folder (it will be in a folder called "Startup", but could be installed at the user or system level) and remove it from the Startup Folder.
Reboot in X and no more kernel panic.
[Editor's note: As detailed on MacFixIt, 10.1.5 appears to have changed something regarding the keyboard handling. Nearly everyone I've spoken to who had installed any sort of keyboard modification kext experienced a kernel panic after booting the 10.1.5 update. DoubleCommand has a new version out which claims to fix this problem, although I haven't tested it myself.]

