I have same situation after upgrade to karmic. IMHO problem couldbe related to KMS which is by default enabled in karmic. KMS defines maximum virtual resolution to 4096x4096 (You can see it, just run xrandr in terminal). Unfortunatelly compiz cannot support this resolution for i945gm - there is hardware limitation to 2048x2048. After connecting external monitor, kernel or xserver is trying automaticaly setup both screens side by side what exceeds maximum value for X).
I have same situation after upgrade to karmic. IMHO problem couldbe related to KMS which is by default enabled in karmic. KMS defines maximum virtual resolution to 4096x4096 (You can see it, just run xrandr in terminal). Unfortunatelly compiz cannot support this resolution for i945gm - there is hardware limitation to 2048x2048. After connecting external monitor, kernel or xserver is trying automaticaly setup both screens side by side what exceeds maximum value for X).