pcasciola wrote:
Are you saying the distance information from the lens with either of these bodies with work with any EX flash, in which case the only feature about the 580EX that is specific to the 20D or 1D Mk II is it's ability to zoom the flash to match the effective focal length based on the CMOS sensor size?
The 580EX also reports color temperature back to a 20D or 1D Mk II to assist the camera when AWB is set. I have no personal experience whether or not this makes a big difference.
The 580EX also sports a 25% faster recycle time than a 550EX, a slightly smaller form factor, and improved user interface.