Too much text in the rest of this thread so I haven't read it, but:
On 8.1.2012, at 0.20, Sven Hartge wrote:
Right now, I am pondering with using an additional server with just the shared folders on it and using NFS (or a cluster FS) to mount the shared folder filesystem to each backend storage server, so each user has potential access to a shared folders data.
With NFS you'll run into problems with caching (http://wiki2.dovecot.org/NFS). Some cluster fs might work better.
The "proper" solution for this that I've been thinking about would be to use v2.1's imapc backend with master users. So that when user A wants to access user B's shared folder, Dovecot connects to B's IMAP server using master user login, and accesses the mailbox via IMAP. Probably wouldn't be a big job to implement, mainly I'd need to figure out how this should be configured..