Ant, I use a GEM rucksack if I'm out and about with my stuff:
http://www.amazon.co.uk/dp/B001FF8TKC/?tag=cliospnet01-21
Looks fairly gimp but pretty inconspicuous, plus I'd rather have people thinking 'hey, there's a bloke with a gimpy rucksack' than 'hey, there's a bloke with a cool looking camera bag. He probably has a couple of £k's worth in there...' Lol.
Of course, the illusion is ruined when you attach a tripod to it.
Regardless, I'm well impressed with it, it's plenty padded enough and has loads of room/compartments. Ignore the fact that it says 'for Canon', I shoot with Nikon and have had nae bother.