Mcgyver and DarkPenguin,
Note that I said I hadn't discovered any better autofocussing of the 40D. I don't often shoot moving targets. A few weeks ago I took a few shots of surfers at the Gold Coast using my 40D with 100-400 zoom in AF servo mode. I wasn't entirely happy with the focussing. On the other hand, I didn't keep changing camera bodies between the 40D and 20D and the 5D so I could compare the rate of hits and misses. Maybe I'll do that some time.
My main gripe about the autofocussing capabilities of the 40D might simply be due to a miscalibration of the one lens which was the main reason for my getting the 40D in the first place, the EF-S 17-55/F2.8.
I returned the lens to Canon for calibration, but it seemed no different after they had apparently calibrated it. I was about to send it back a second time when I discovered that the nature of the focussing target changed the result.
In order to get this lens to focus on the wooden figurine in the image below, I had to tape to it a small cut-out containing contrasty text. Then both the 40D and 20D could autofucus accurately. Without that B&W text, neither the 40D nor 20D could autofocus accurately. They were both equally good or equally bad, with or without the text.
I have the 40d plus 17-55. Focusing was terrible(front focus) until I returned the lens to Canon for adjustment. After that the combo focus is very sharp in all conditions. I sold the lens last week however since I found the flare to much to my taste.