This is a near perfect solution. I run an Asterisk Vo/IP system for my office, and eGroupware with Cyrus for our mail system. We migrated from Microsoft Exchange some time ago and I've never looked back.
With eGroupware, we were able to replace 99% of the functionality of our Microsoft Exchange server with open source software. With sieve filters users can set up an unlimited number of server-side mail filters, something you can't do with Exchange anymore, and the web interface works much better with Firefox than Microsoft's web access. Our Asterisk phone system integrates into the mix and will deliver voicemail to the user's e-mail box.
Now, with my Blackberry I can download and play those voicemails without ever having to dial into the phone system, because they are wav files attached to e-mail that is picked up by my Blackberry. I'm feeling so "connected" right now it's not even funny :-)
Switching to an all open-source messaging suite like this was easier than I thought, but when we did the primary migration I only had about 10 users to worry about. This made it much easier. I took Outlook (or the appropriate mail client) and downloaded everything to a local storage. After setting up IMAP to the Cyrus server I pushed it all back up - not recommended if you're going to migrate a few hundred users, but effective in my case. I then set up the Funambol open source sync client for each user to sync their contacts and calendar between the server and their local store. This gives them remote access to their data, and I have a copy of that on both the desktop and the server in addition to the server backups in the event of a catastrophic failure.
On my Blackberry I again use the Funambol client to sync contacts and calendar to the server. Within 30 minutes of getting the Blackberry I'd already downloaded the Funambol client over the air and sync'd with my eGroupware server, without ever attaching the Blackberry to a computer.
Configuring Asterisk to deliver voicemails as a wav file to your e-mail box is simple. I now I have a completely integrated messaging system that doesn't tie me to a desktop computer, and every aspect of it is open source software!