I now have an IMAP view of my mail spool and procmail-sorted mailboxes. I built mutt on the G4 to test it out, and it runs flawlessly, it's just like I'm ssh'ed into b5 running it locally. It probably won't be so fast if I ever run it on the iBook from a remote location--mutt caches nothing, but I'll evaluate that later.
At recursive's suggestion, I also gave Apple's Mail application a whirl. I'm actually quite impressed with it. It gives me the same view of my mail that mutt does, with the nice graphical interface. It even lets you pick IMAP folders to use for its draft, sent, and junk mail folders, rather than storing them locally.