We're using Cisco Unity Connection voicemail, and I got the Zimbra web client configured to download voicemail as an external account. It works great.
Unfortunately, when I try to use my iPod touch, it doesn't download voice messages. It will download any email message sent to that voicemail box, but that's it.
I have a suspicion it has something to do with either the way the iPod retrieves the mail or the ActiveSync process.