I used to have problems with a mobility Radeon on my Thinkpad T40 a couple of years ago but things may have changed
I currently have Ubuntu Hardy on a Thinkpad T60 with an ATI x1300. compiz ran out of the box. Have not had issues so far.
The first time, it's a KLUDGE! The second, a trick. Later, it's a well-established technique! -- Mike Broido, Intermetrics