We have run into a visual error with my directional shadow maps when running on a Radeon 5750. It's hard to explain, but the edges of the shadows appear very blocky. In order to verify that it wasn't our implementation, I checked the Radeon SDK ShadowMapFetch4 (http://developer.amd.com/downloads/ShadowMapFetch4.zip) and it displayed the same visual error.
If I had to guess, I'd say it isn't actually fetching 4 depth values, but instead is returning a single depth value in r, g, b, and a.
We've tested it on 2 different systems with the 5750 and both showed the error. One system was a Windows Vista 32-bit and the other was Windows 7 64-bit.
We've test it on various other Radeon GPUs (x1950, 4650, 4850) and haven't seen any problems.
Could this be a driver issue or possibly something more sinister?