I have just confirmed with a direct test that Dovecot 1.0.0 using Maildir does properly service 2 simultaneous IDLE sessions on the INBOX. I've got my Palm ChatterEmail client logged in and idling and have a manual IMAP session (telnet localhost 143...) and the messages are popping up on the Palm in lockstep with the 'EXISTS' messages kicking out on the telnet session. Meanwhile, Eudora is doing its things from my Mac against the same mailbox (actually tossing messages out of INBOX to other folders) and I'm pretty sure I left Outlook running on my office machine and looking at the same mailbox, so whatever flaky stuff it does is happening on whatever schedule it uses as well...
Thanks. I have been using the same setup as you with no problems on my existing server and loved it but have to say that the future of Chatter looks questionable and my BB's get much better voice and battery.
Note that one strong consideration for IMAP servers with more than a handful of users is going to be dealing with the high concurrency of a lot of eternal connections from the Blackberry server and from other clients latched onto the same mailboxes. I suspect that one reason some systems limit concurrent sessions is that they don't have (or haven't be configured to use) efficient filesystem event monitoring, and the alternatives are quite burdensome. In the case of Courier, it looks like it still relies on SGI's FAM, an abandoned project that doesn't really work well anywhere but Irix and Linux. Dovecot has avoided that at the cost of having to understand each system's unique approach to that issue.
Thanks for the insight.