The Tamrac Aero 85. It's got a fairly sizeable camera section, with access through the side, similar to the slingshot (I think), a decent laptop section and another section for lunch.
http://www.warehouseexpress.com/buy-tamrac-aerospeed-85-rust-rucksack/p1030179
Even my wife quite likes it...
These are amazing backpacks for the price but I've just done a search on Amazon and they seem to be currently unavailable. If they come back in stock then you can't beat it for £30.
http://www.amazon.co.uk/exec/obidos/ASIN/B001CSQE8G/ref=ox_ya_oh_product
The crumpler doesn't look that good for access. If you had a camera with a lens on it, would you not need to take it all off to take the camera out safely?

Unless you're talking 'slingshot' style I dont know one backpack that you don't have to take off to 'safely' access your gear![]()