Seems like the problem is really related to said graphics chip (Intel 945GM) in combination with >=4 windows opened... I'm running Debian Jessie on my Lenovo T60 and the described behavior is reproducible here also... The only way to get everything back working again is by switching to a non-X session (Ctrl+Alt+F6 for instance) and restarting the gnome shell by executing DISPLAY=:0 gnome-shell --replace & as root... this works sometimes, sometimes it doesn't due to display 0 not found or something... that's the workaround I'm using currently... however, this is not really convenient as it really happens quite regularly and as mentioned already, the workaround doesn't always work properly... in case there's anyone with a better solution feel free to tell me... always interested.
Oh, btw. the crash happens not with an external display for me but with the internal one which is quite high resolution (for the time the laptop was released) with 1400x1050 pixels... so it really seems like the graphics card somehow runs out of memory and that's the result... strangely, I never experienced said issue when I installed the latest Ubuntu (13.04) and gnome 3 (however it was 3.8 I installed there) on top... only downside: it was waaaay slower and bloated with ubuntu crap noone needs ;)
