Comment 79 for bug 660596

I forgot to mention that I have two graphic cards in this system which might be the cause of the problem:

$ lspci | grep VGA
00:02.0 VGA compatible controller: Intel Corporation 2nd Generation Core Processor Family Integrated Graphics Controller (rev 09)
01:00.0 VGA compatible controller: nVidia Corporation GF106 [GeForce GT 555M] (rev a1)

Actually it's not 555M (I wish it was), only 540M.

I also found bug #643895 which is a similar problem.
In my Xorg.0.log I also have:

(EE) No devices detected.
Fatal server error:
no screens found