[curiousuburb]

ok... let's try asking the obvious...

do you have Filevault enabled ?
(this would account for regular activity as it encrypts/decrypts on the fly)

is network activity simultaneous with disk activity ?
network threads (including rendezvous announce/discovery) process higher than many others and can account for some drive activity at times when no indicated application seems to be running.

P2P running?
Acquisition will stream info about your shared files to other users (ultrapeer settings or not), and seems to occasionally check not only for incomplete files getting moved into the active directory, but also scans subdirectories for newly shared files... you'd probably see it as a process, but if all you notice is network access and port 63xx traffic, I'd suspect P2P.

Mail running?
Apple's Mail program seems to occasionally check the address book and index mailboxes (particularly when iChat is open), despite otherwise running as a background process.

Scheduled events?
Software Update calling home? any other apps checking their mothership for updates (Acrobat, etc). some have a specific menu or preference item for frequency of update check...

once we can categorically eliminate these as causes, the troubleshooting may get easier