Hello,
The solution proposed in
https://bugs.launchpad.net/ubuntu/+s...er/+bug/186354 worked for me: disable xinerama.
On ubuntu 9.04 (using an Intel GMA chipset), I added this at the bottom of my /etc/X11/xorg.conf file and then had no more problem after restarting the X server:
Section "Serverflags"
Option "Xinerama" "false"
Endsection