Well, i currently have a canon 20D and will be getting a 40D within the next 2 weeks.
Main reasons for me to upgrade:
Better AF points
Custom setting (so if i scout a location i can instanltly switch back)
Better Noise reduction
and generally a better upgrade than the 30D
I was going to save for the 5D replacement but to be honest, unless i get a few more paid photography jobs, it is out of my price range for now.
Would be good if someone on these forums did a full review of the cameras - if we had a format to follow certain things to look for, shoot comment on, then we could start a camera review section here - as we all have different cameras and different lenses, im sure it wouldnt take long to build up a pretty comrehensive list of products that we can review.