I'm currently in the process of migrating to Zimbra. I had to export some Palm users to Zimbra. These Palm users use PalmDesktop to sync their calendar. I'd to export that calendar and import it to Zimbra so all appointments would be migrated. The Palm users get a new mobile (Palm Treo 700w) that is using windows mobile, so I did not have to make them sync with Zimbra.
Unfortunately PalmDesktop doesn't allow to export to any other file-format than it's own address-book format.
Searching in Google I found a plugin which makes it possible to export to ics. I'm posting it here so anyone with the same task can save some time searching for it:-)
DIMEX 3.0 Desktop Add-In for Calendar Import/Export
It's a $19 dollar shareware and well worth the bucks. You can try it for free.
I imported the ics file using the REST interface described in the wiki. Works great! :-)