Hi Thomas,
I had tested this issue before:
In my lab (Exchange 2007 SP3 and Outlook 2007 SP1):
The Authentication for POP3 and IMAP4 services are : Plain text logon(Basic authentication)
After I enable protocol logging for pop3 and imap4, I can see the information of Data-time, Connector-id, session-id, sequence-name, local-endpoint, remote-endpoint,
event, data and context are all included in the logs, and find the users who login under the context.
Here is a related thread for you, hope can give you some help:
how to show current logged users using imap protocol
http://social.technet.microsoft.com/Forums/en/exchangesvrmonitoring/thread/4b566940-4571-4a50-b473-0dcf40bdc446
Thanks,
Evan