The bug that has kept me awake till 5AM is here
It happens by itself, I wake up to a gdm prompt when there was a full session the night before.
It also happens when working, but never repeatably, e.g. open libreoffice BOOM. blank screen, then gdm prompt.
Tried different window managers, now different nvidia drivers, different kernels (from .34 to .38). It's always there, in both laptop and desktop (!). Maybe software has something to do with it? I use xflux, which may be doing something to all X windows... no idea...