Update - I switched to a recent build of the open source (radeon) driver as part of upgrading to Intrepid Ibex Alpha 5 yesterday, and the 2D and video performance seem better than using fglrx... at least on my Thinkpad T60 with X1400 (RV515) with EXA enabled, AccelDFS on, and DMAForXv on. Xv implementation is definitely better... there are more ports available and media applications can autoconfigure to Xv, unlike under fglrx 8.7 when I had to explicitly specify an Xv port.
It looks like Bridgman has been talking good sense when he has recommended the open source drivers for those who doesn't need the advanced 3D performance specifically. It's truly a more simple solution, and one that seems more tightly integrated with the OS, at least in Ubuntu.