After you download the (German) iBookScreenEnhancer script, run the first script. It will check whether your graphic chip is capable of supporting an external monitor or not. You can then enable (Press 'Ja') or disable ('Nein') the feature. Afterwards, un-check the checkbox 'Monitor Synchronization' in the preferences and there you go.
Some remarks: If anything goes wrong, press Apple-Alt-o-f while turning on your computer to reset the Open Firmware. It's supposed to work on other models (eMac, iMac...) too, but not yet tested.

