The same problem with Kubuntu 7.10
Card Radeon 9100
Temporary solution :
1. CTRL+ALT+F1 (go to console)
2. ALT+F7 (go to X org)
Driver (?) didn't set up proper resolution/refresh rate ,
when changed in display settings (even to the same param.)
after confirm to change resolution screen goes black and monitor turns off.
Want more information?