I've had the same issue as Andreas - I have had freezes while using MergedFB and not using xrandr, but if I do use xrandr to resize the screen (two heads -> one head -> back to two heads) it will freeze consistently.
-- [dapper] xrandr freezes the system (radeon, MergedFB) https://launchpad.net/bugs/47775 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
