A great lowepro bag that many of the forum members use is the flip side 200. here you can see an excellent user review by Daniel...
I own this bag and would highly recommend it if security is of any concern to you. because of this bags design you can only open it by dismounting the backpack. check out the review link above to view pictures so you can see what i'm talking about. i recently traveled to Manhattan and while on the crowed subway i felt fine with my camera on my back knowing nobody could steal anything while it was on my back.
hope this helps