But then I changed desktop to Gnome (Unity sucks, gnome 3 also sucks but not that much, this is why I stayed at Ubuntu 10... that and the memory usage is way lower at 10...) and installed compiz extras to enable Wobbly windows. Right after I enabled wobbly windows, the guest locked up and I had to reset it (!). When I booted it next, there was no GUI. It shortly flashed the GUI (0.1 sec) and then crashed. When I log in on the command line, I can use "startx" and I do get a GUI with wobbly windows... I started ccsm and disabled wobbly windows and again the guest locked up, forcing me to reset it. On the next boot, GUI was back as normal.
It seems VirtualBox cannot handle wobbly windows anymore...