IMAP folders would be more powerful if they could serve collaboration as well as personal storage. For example, it would be nice to be able to setup an IMAP folder for planning a company picnic and whenever I drop a message to that folder it would be shared out to the rest of the planning team.
One way to accomplish this would be to follow a dropbox approach. A separate web service is configured to read your IMAP folders and poll them for new messages. Once a message (or multiple messages) is found it is copied to your collaboration tool. For example, it could be copied into a BaseCamp or activeCollab message. When the copy is complete the toll could then move the file into an archive folder indicating that the message has been transferred.
Another approach would be to hack the Twisted Server IMAP framework and provide dynamic folder subscriptions along with user group permissions. This would be preferred but there are issues with port numbers and client configuration.