After playing with my own configuration, I realize I'm actually somewhat closer to a working prototype than I thought.
First, when configuring the SyncML client, set your username to the full account on the server - i.e. "user@zimbra.foo.com", rather than just "user". This will ensure that you have permission to create and modify events on the server.
Second, when doing the initial sync, start with a 'recover' operation if at all possible. The Funambol SyncML client for WM2003 supports this - I'll try others. The 'recover' operation instructs the client to discard everything it has and grab everything from the server. This will prevent it from trying to match up what's already on your PDA and will ensure that the client and server are in sync to start with. After that, 'fast' syncs seem to work reasonably well.
I'm still having a problem with timezones though - when you change the time of an event on your PDA, it will treat the new time as a GMT time, usually resulting in your meeting moving to late night/early morning on the server.
So my prior warning about it being "highly" experimental is perhaps a bit harsh - it's just "experimental". If you have the inclination, and have a good backup of your calendar data, feel free to try it out with other PDAs. I would try it on a test account first though
