If you like Ubuntu, but just want a light distro, try WattOS. It's an LDXE-fronted version that uses the standard Ubuntu repos.
I've been using it on my original EeePC 700, and it's quick and comfortable.
You want... ::Rolls Dice::... wattOS!
>wattOS is a lightweight Linux operating system remastered from the core Ubuntu Linux build. It is a free operating system that focuses on a small footprint, low power, and a simple quick interface. Bring your old computer back to life again with a fresh install of wattOS!
Ok, I didn't roll the dice. I got a random distro from distrowatch.org.
Hi, thanks for your answer. I tried a lot of combinations but nothing worked. Though, I did find a solution. It seems to be a very old problem that has nothing to do with Qubes but Xfce. Here is the source for the solution:
http://www.planetwatt.com/forums/topic/77/xfce-power-manager-does-not-swit
In short:
1.) disable the display power management
2.) go to Screensaver settings and enable the display power management
3.) set the first two options (Standby and Suspend) to 0 minutes and the third option "Off" to one or more minutes. My display switched off after I set it to 1 minute and stayed off (backlight as well)