Cannot exit X server, restart, shutdown or drop to tty when VGA monitor active

Solution 1:

This Ubuntu PPA has a more recent version of the nVidia closed-source driver. Alternatively, you can try nouveau, the open source nVidia X driver.

Solution 2:

Well, LMDE Update Pack 5 came out yesterday. The problem was fixed when I updated my system. I don't know precisely which package was fixed but my current specs are:

LDME, kernel 3.2.0-3-amd64
Xorg: 1.12.1.902-1
nVidia kernel: 302.17+1+2+3.2.21-2

Oh well, all's well that ends well. On the assumption that it was the nvidia driver update that fixed it (though it may just as well have been xorg or the kernel upgrade) I am awarding the bounty to Stefan who needs it more than harrymc :).