But I'll assume that Darwin is fairly modern and pages instead of swaps and that that's not unreasonable.

Yeah, that isn't terrible. I have had a dozen or so going on my work computer when doing java development.

But I was using the thing last night running nothing besides a browser beyond the normal iBook processes and it was constantly grinding the disk. It was incredibly slow. I never encountered this running a single application under 10.1

Hmm, that just doesn't sound right. Here is a few things you might want to check... Is it possible that you've got something else running that is grinding the hard drive besides swap? Have you ever installed Norton Utilities, Virex, or some other evil maintenance tool that likes to do things in the background? Does 'top -u' show anything burning a lot of CPU? Are you running out of hard drive space (less than a gig or so free)? Do you have anything running at startup (/Library/StartupItems) that you don't need? Have you tried booting single user (hold down Command-S) and doing a fsck?

Also, consider running a cache cleaner like this one to delete any temp files that may be hanging around. If you use the deep cleaning mode it will also delete kernel extension (kext) caches and other random things that could cause problems if corrupted. I have only personally used this particular tool once so don't blame me if it does something evil.

-Mike