I'm sorry to dig up an old thread here but I came here looking for answers. I had the same problem, I couldn't start any GUIs from a terminal. I'm running OpenSuSE. I did manage to find the problem though.

If I start a regular terminal window and run su to switch to root, I can simply run:
Code:
export DISPLAY=:1
Where :1 is my X-Windows display.

However, if I start the terminal window with...