I am running Ubuntu on a ThinkPad X100e. This page documents various issues and workarounds.
Lockups when adjusting the screen brightness (Lucid Lynx)
These happen when you hit the keyboard buttons, when resuming from suspend or when switching between mains and battery.
Resolution: Boot the kernel with the option radeon.modeset=0. To make this permanent edit /boot/grub/menu.lst and add radeon.modeset=0 to the "#kopt" line, then run update-grub.