I really like the Tamrac Velocity series. I have used a Velocity 7 walking around Rome, on safari in Uganda, skiing in the Sierra, etc. It can ride on one shoulder, very casual or across the chest (very secure for sporty things). There's a waist strap which I click together if I don't need it, but other times it is very essential.
There is room inside for Canon 20D with up to 300m lens plus a second lens. There's a zip pocket in the lid for batteries & cards and a larger zip pocket that could fit a flash on the exterior. In a pinch I can stuff a light jacket under the lid. Also I bought the Tamrac accessory for water bottle which I mount on the side of the pack, but that's optional. (Just don't like the thought of a water bottle inside with my camera!)
I modified two things on this pack. Sewed a patch over the Tamrac logo to disguise the bag, and cut off two side fastex buckles which are unnecessary. This leaves one central buckle to undo for very quick shots: swing the bag around to your front, unclick one buckle and shoot at will. Putting the camera away is just as fast. I'm able to ski with friends, get their photos and keep up because the of ease of use with this bag.
my 2 cents,
Andy
http://www.pbase.com/lighthawk