In the words of Ned Flanders parents, "We've tried nothing and I'm all out of ideas", except I've tried everything!
I can use the number keys on my extended (wired) keyboard, but only some of the numbers, other just have no input response at all. All number keys along the top work fine. Probblem persists in all apps so I guess it's a system prob.
Tried mouse keys in Universal Access, tried new KB, tried KB on other Mac (works fine), downloaded and applied 10.4.10 combo updater.
Interestingly, if I run the keyboard identifing sys pref it tells me I am using the internal keyboard and to attach an external one. Maybe the Mac (a G4 tower) thinks it's a laptop?