So, after cleaning up a bit, I did an "Archive and Install" on my iBook. Despite wiping most of my custom preference panes (expected) and iLife'05 (completely unexpected) and taking a couple of hours to index everything, Tiger installed fine, and pretty much everything was where it was supposed to be when I logged in.
Tiger is, however, a battery killer if I ever saw one. Not sure if it's the 6GB of e-mail Mail.app is trying to sync with my IMAP server or Spotlight greedily peeking into anything I drop into the disk, but I can see myself spending some cash on a new battery pretty soon.
In the meantime, Mail.app is really getting to me. The buttons are hideous, and I can't seem to be able to reproduce my earlier vertical preview pane hack (the .nib files were subtly changed, and I've yet to figure out what else I did besides changing splitter orientation). Plus, syncing .Mac Mail.app settings apparently doesn't sync the "Use this folder for..." (Trash, etc.) choices.
Still, that's progress for you.