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Nono pointed out that the 20D too, supports rear-button focusing (RBF)! This means that you can use the AE Lock (Exposure Lock) button [at the back of the cam] for focusing, and not be forced to half press the shutter button.
I’ve then since added some information about the function to my previous post.
This has a number of obvious benefits – mostly during sports/events photography where people prefer tracking using AISERVO. But in a practical sense, RBF is great as you can take your shot regardless if you’re focused or not. Normally, if you’re on AF (auto focus mode), you’d have to focus by half pressing – and as long as it’s not yet focused, you can’t trip the shutter.
This is a sensible approach, but can be frustrating at times: If you focus on a subject, and know that you’ll be taking another shot of them (assuming you’re not on continuous drive), you’d still have
to half-press and waste time as the camera re-focuses on [probably] the same point (which is ridiculous if you think about it)
RBF allows you to trip your shutter regardless, so it’s a kind of pseudo AF/MF focusing mode, which I think is pretty cool and useful.