I just wanted to second NHL's comments (like I often do.)
The range of a flash in HSS is not nearly as far as you might think and I'd be very surprised if it matched the 25 yards that JohnG mentioned.
Also, the way the flash works is different depending on which mode you are in. In Av the flash is assumed to be a fill flash and therefor uses less power... this means it will recharge faster but might cause trouble when in the red zone (like JohnG said, there is often less overhead lighting there.) In other modes (P, Tv, M) the flash is assumed to be the dominate light source and the camera tries to use it at a stronger power. If you really want to do that, you might want to consider getting a battery pack for the flash as it would enable much faster recharge and make it recover faster from larger flash bursts.
From what I read, which is limited, the 30D's AF is the same as the 20D. But quite honestly I purchased the 1D MkII N earlier this year and now use my 20D as a backup. I haven't looked that closely at the 30D but I've read the AF system is the same.
I've heard varying debate about how Nikon can work in focus-priority or shutter-priority mode (higher end cameras have a custom function for it.) Canon can't. Some want Canon to add this custom function, but from what I've read it won't happen.