I'm using xubuntu 8.04.
Nether xlockmore, or xscreensaver's screen locker work. I type in my password and I get "Authenication Failed".
I have to Ctrl+Alt+F1, then login (which works), and kill the process off.
Also, xscreensaver seems to be crashing my display driver after between half an hour and a few hours, but that's an issue for another day.