Laptop constantly goes into sleep mode

Lemerise, thank you so much -- that was the problem! You've saved my iBook! :-) I pulled the reed switch cable, and it now works. Oddly, I plugged in a different one, and it had the same problem, but I'm happy to just leave it out and have a laptop that doesn't fall asleep when the lid closes. It's funny, I'd always assumed the reed switch was something to do with Bluetooth, since this is the only 12" with Bluetooth. Now the problem with the machine is that the screen stays black until the OS is almost completely booted (i.e. you don't see the Apple icon phase), so I don't know what that's about...I tried another screen with no change. But hey, at least it works. Thanks again!

On the 12" 1.33 model the bluethoot hardware is integrated in the airport card and the two antennas cables goes from the airport/bluethoot card to the display via the iBook hinge. There are other iBook G4 models that use bluethoot (the 1.2Ghz I think). On these models the small bluethoot board is screwed on the metal frame near the top case cable socket and connects in a socket near by.

Try to boot the iBook from another source (without ram installed) to check if you'll have video onscreen when the boot process begins. If you get the same black screen then it could be a bios issue. Maybe there's a firmware update that could be applied to the iBook, I don't know. Anyway if the iBook boots and runs stable then the logic board is probably OK.

I tried a third reed switch, and that one worked fine, and I tried another screen assembly, and somehow that resolved the "black screen until OS has booted" issue, so the laptop is working fine now. I guess it goes to show that it never hurts to have tons of spare parts around. Thanks again!