PHP SyncML Library
It seems that egroupware (a PHP based collaborative tool) is integrating horde syncML library, so that they can synchronize contacts, calendar, etc… with external sources (i.e. Exchange Server). The next 1.2. release will contain the source for that.
IDEA: It would be interesting to integrate ILIAS calendar with external servers.
Ah! my Siemens CX65 seems to be SyncML compatible 
Synchronica Gateway
It seems to exist a commercial gateway to exchange servers, have a look.
Scalix
Looking at this Exchange Server issue i finally found an open source solution that i didn’t know: Scalix, Enterprise email and calendaring for Linux.
