Small update:

It seems that N900 calendar and task RE-sync will work only one-way - all calendar management needs to be done in Zimbra. When editing calendar or tasks in N900 then calendar resync wont work anymore (sync stalls until activesync daemon timeout) - to fix it full calendar sync needs to be perfomed (sic!).

The biggest change from Zimbra v5 to v6 and N900 sync behaviour seems to be that now full calendar sync seems to work - with Zimbra v5 it did initial full sync but exited with unsuccessful status - now with Zimbra v6 full sync completes ok.

But it seems that calendar two-way sync is pretty much broken - so MfE is still not very usable in real life situation.