I read through the thread on Stale imap process and would like to chime in that I too am having the problem. The only imap user on the system is using Mail.app 2.1. The other users are using pop with thunderbird. After quiting mail.app and stopping dovecot I can still see many imap sessions opened. Over the course of the day a good number of imap process will be open and no one will be able to connect to the server via imap or pop. Typically I have to restart dovecot twice a day with the following:

I've narrowed it down to an imap issue. If i don't connect with imap the other users don't report any issues connecting with pop.

I'm using mbox as my mail format on centos 4. I know that the default version on centos4 is .99, but it appears that people are still having issues into 1.0rc2.