unwanted logout in xfce

[Stuart McGraw]

I am a refugee from Fedora 15's Gnome-3 that has found 
refuge in Xfce.  I'm am only sad that Gnome-3 didn't come 
out sooner so I could find Xfce sooner -- it meets my needs
and expectations very well.

However, I am having one issue since I changed... I leave
my machine on and running overnight (have a bittorrent 
client running) but the next morning I find I am logged
out.  This happened again last night.  According the logs 
there was no reboot.  'last' is hard to interpret but I 
*think* it shows I left the machine around 22:56 last 
night and was logged out at 23:33.  

I have not been able to find any setting that might be 
responsible for this.  Any suggestions about where to look?
(This is a desktop machine and power management is set
to "none" on external power.  Screensaver is set for 30 min
but not set to logout or even lock screen and test this
morning indicates it is not culprit.)