For the weddings I've shot with a 5D, 20D or 30D, the one thing I had a HECK of a problem with is the processional/recessional shots. I could never get a reliable system of shooting these events down. The 5D was horrid at tracking, especially in low light. Servo mode was very unreliable. I tried one shot using centre point but it either required me to either prefocus on a spot on the aisle, or use a smaller aperture to increase the DOF. Neither worked well because people always seemed to blink at that prefocused spot, and with such low light, stopping down is usually a poor option. Recessionals were even worse as they usually move a lot faster.
Thankfully, there are absolutely no worries now about this with the mark III bodies, which can handle these events like a walk in the park. They also provide me with a MUCH higher keeper rate for dance floor shots using wide open primes. To me, focusing was always the Achilles heel of the 5D. Strangely enough, between the 40D, 5D, and mark III bodies, I found the 5D to be the least satisfying to use. Image quality was superb, but operationally it wasn't enjoyable. Which is precisely why the 5D it not in my lineup.
I'm not saying it's NOT capable... but for *my* style of shooting, and for what *I* expect out of a wedding body, the 40D and mark III bodies proved to be far superior. Many others would disagree totally. 