These days ATI as well as NVidia are far too slow in adapting their Xorg drivers to new Chips as well as to new releases of Xorg, the Linux Kernel, and Distros. 
Therefore everyone who prefers e.g. working suspend/resume over GLX gimmicks, tends to go back to the VESA driver - which unfortunately tends to make it difficult to run "unusual" resolutions, especially for wide screens. (VESA ist still discussing which 16:9 and 16:10 resolutions they are going to declare official) 

So here are the steps that gave me 1920x1200 with VESA on my ThinkPad T61p:

This did it for me ;-)) 



From: IBCL BLog.
Originally posted: 2007-09-14