Maybe, I am missing something, but I presume that adding support for IMAP IDLE server command (just like Apple did starting from Leopard's (if memory serves) Mail.app) would be just great and won't require checking mail servers at any regular intervals at all.
Just want to voice my vote for this one
I also would love for this feature to be implemented...
Currently messing around in the source to see if I can provide a patch.
I don't think this is all that simple to implement, but I'd be happy to integrate a patch if someone comes up with a solution.
By the way, I've been experimenting with the possibility of replacing MacBiff's IMAP classes with MailCore, an open-source framework that includes a lot of the features that people have been asking for (including IDLE as I recall). This is probably quite a long ways out, however.