This was weird. I offered to upgrade a friends QS 733 with Apple 15" LCD to 10.3.3 and all was going to plan. We'd checked the disk, repaired permissions, installed update all without problems. Restarted the Mac and the keyboard and mouse wouldn't work. It was failing to recognise the USB sockets on the back of the screen. Restarted to no avail but luckily after another repair of the permissions all started working again. Bizarre...
PS I thought this belonged in the OS X section as it was 10.3.3 related.