I'm running the open source version of Zimbra and connecting to mail on my Motorola Droid through IMAP/SMTP. The only thing I really miss is a CalDav client. So far I haven't been able to find a CalDav client anywhere for Android devices. With all of the applications being developed right now you'd think someone out there would make one free or paid even. It's working great with my Google Calendar but I'd really like to get my Zimbra calendar in the native Android calendar as well with read/write access using SSL login for CalDav. The way that Iphone/Ipod Touch does CalDav access works great with Zimbra and I wish we could get this soon for Android!
Currently issue number 14 on the Google Android code issues page.
Issue 2361 - android - Add support for external calendars via iCal / CalDav - Project Hosting on Google Code If you would like to get this added to Android OS then go to the Android code site and STAR the issue so it gets more attention.