I taught some wedding pros Advanced Flash last night. I used my 7D:

And I was struck again by one major reason I bought a 7D: I can now, just like the Nikon users, drive my external flashes from the popup. That means my 580, which used to sit on top of the camera, is now my off-camera key light instead.

This is pretty major – it reduces the price of the 7D by $500+, because essentially I now get an extra 580EX flash free of charge.

And you can set separate ratios for built-in vs. external, and much more. This is a very major overhaul of the flash system – I can now use the 7D whenever I uses multiple strobes, since the 1D MKII and 1Ds MKIII do not have a popup flash. And no other Canon offers the option to drive TTL flashes from the popup.