Hi Marcelo!
Sorry but I can't reproduce it anymore. Yesterday some updates had been installed to the system. Now I doesn't get any backtrace in Xorg.log.
The "Problem" has also changed a little bit. If I switch to the Console and then back, it takes about 1 minute and the screen gets back. (thats better than before :-D )
I also noticed that if I disable the "Desktop Effects" in System Settings the Problem disappears completely.
Peter