I've just gone into town and picked up my Lowepro ProRunner 350.
the only difference in it, from the 300 is that it has a laptop slot at the back. Use it or not, but it provides another layer of padding between camera and back.
So far i've fit in it:
7D with 70-300L and hood in place, stealme strap with RC6.
Canon: 15-85, 50 f/1.8 ii, TC 80 N3, battery charger and cables.
lensbaby: control freak, 0.42x WA, 0.6x WA, 1.6x TC, extra optic, macro lenses in pouch
kenko: EF/EFS extension tubes
Other: gorillapod SLR Zoom and Cullman Ballhead, Skink pinhole, some sort of 52mm screw-in fisheye adapter, gitzo blower. Step-up/down rings and reversal adapters and extra cards in the front pocket.
All of that is 7kg, and man it's comfortable, i'm about to go for a walk to try it out, it's already better than my Toploader stuffed inside my other Black Wolf backpack i bought for uni work.
I did see those Kata (i think) bags with the hole in the side for easy-camera-access. it's agood idea, but i didn't like them
a) because of the security already mentioned
b) because it was on the side and horizontal, you limit the max length of lens
c) if it was on the top and vertical, weight is higher up which is better for my back