Just noted something wired.
Having three address books:
- Default
- Frequent Contacts
- Test
Having default settings in the GroupWise part of the Data Synchronizer Connector, new contacts on the mobile get created in 'Frequent Contacts' which is not selected.
Now I changed the settings, such that Frequent Contact and Test are selected, synched, create a new contact on the mobile. Guess what, the Contact got created in the Default (again not selected) address book...

For the other direction I have some trouble, but currently I blame these to my Motorola Milestone...