I'm sorry that I did not post this earlier, but as I just
reported I've discovered a potential work-around for the failing 'Xorg' when using the VBox additions: Just replace the 'waitforX' in '~/.xsession' with a suitable 'sleep ...' command (e.g. 'sleep 2').
I believe there are still some further issues with the VBox additions, e.g. the mouse integration does not seem to work quite right, but as I'm already on record not to be using the VBox additions on a regular basis I'm unlikely to track this one any further.
OTOH as VBox 4.0 is now out the question really is whether it makes sense to get to the bottom of issues with a now slightly older VBox version (i.e. 3.2.12).