I was able to copy -- via PCMCIA, which stands for "Really Really Slow" in Arabic or something -- all of my main system partition and the important files from my source partition to another drive (12GB I think), and then stuck that drive in the ol' PowerBook G3/400. I booted it with the exact same system, and it seems fine, albeit slow. It's no G4/667, but it will do the job until the G4 is fixed.
I am going to limit my work on this machine for the next few weeks to making important upgrades/changes/etc. only when necessary, so I will have less stuff to copy back to the G4 when it returns. My email folder, maybe some source (although I should just be able to cvs update/p4 sync, so that shouldn't matter much, as long as I commit my changes).
Later tonight I'll do a full backup via FireWire to the 160GB external drive, and call Apple to check to see if they have parts in stock, and then send the machine in (assuming they have the parts).