My groupware (egroupware) is a SyncML or XML RPC server.
http://www.egroupware.org/sync
http://www.egroupware.org/index.php?...yncMLSynthesis

I read that gpesyncd is also compatible with SyncML. So I think that it exist a way to sync them directly but I'm not a developper and I dont Know how to set up this solution.