When I connect an external monitor using the DisplayPort connector on my HP ProBook 6550b, it works like a charm. However, when I *dis*connect the DisplayPort cable, the UI on the laptop freezes. Sometimes I can recover by cycling through suspend / resume, but at times this doesn't work either.
I also tried disabling the external monitor first before disconnecting, but the result is the same.
When using a VGA cable to connect the external monitor, everything works fine.
I'm using Ubuntu 11.10 64bit with Unity 2D.
I wanted to file a bug about this, but the problem is I don't know if this is related to either Unity 2D, Xorg or the Intel driver (or something else).