VoiceMail stoarage within IMAP for a voip system has been done before. It's how tekelec's voicemail is stored. The Voicemail app checks the imap email box, and then plays them out. Unfortunately, this voip platform is EXPENSIVE. But I do have to admit, it's pretty darn efficient.
The message contains the information about the message, including time, caller id, and the like, and the voicemail itself is an attachment.
Just thought that I'd offer an "its been done before", but I would like to see this in Asterisk as it would make it a better sell for companies like mine who are impressed with flash.