MBP freezes after disconnecting from projector

Hi to you all,

I've run several times into this same issue.

When I connect my MacBook Pro 15" (mid 2010 model) to a beamer/projector (no matter what brand, I think, but almost always with Dell Beamers) via the display port, everything goes very well. The beamer and its supported resolutions are almost imediately recognised, so I'm up and running in no time (offtopic: always funny to see how much trouble this is when one of my colleagues hooks his Windows Laptop to a beamer/projector).

But when I'm done showing my work on a beamer/projector, and pulling out my cable (Apple Mini DisplayPort to VGA Adapter) from the displayport, my MBP 9 out of 10 times completely freezes. The only way to fix this is to manually reset it (holding down the power button for a few seconds).

This is very annoying (and very funny for my Windows colleagues), and I have no idea why this happens.

How can I fix this?

Thanks in advance.

p.s.: My MBP runs the latest OSX Lion, with all updates available.

p.p.s.: Here in the Netherlands we call a projector a beamer