This might be useful to someone so I'll throw it out there.
Patch against 1.0.5 adds support for two new configuration params:
pop3_debug_client_commands = TOP, RETR, DELE imap_debug_client_commands = FETCH, DELETE
Obviously add in whichever ones you're interested in, or none, or comment them out completely.
It'll log the appropriate events:
Oct 12 18:30:18 kea dovecot: IMAP(zach): Client command: FETCH
The parameters are included for POP3 events. Wasn't sure quite how to do the same for IMAP and for my purposes it's not too important so imap command parameters aren't logged.
Running smoothly here with a few dozen IMAP and POP3 users. Not suggesting this gets added to the codebase - I just needed it to help debug some other IMAP software and thought it might be useful.
Zach.