I'm going to the olympics and I'm concerned about pick pockets and thieves. This back pack was exactly what I was looking for!  This bag is the smaller version and doesn't have the main storage separated into smaller sections, so if you want something to separate thing, go with the larger bag.

Update:
I came back from the olympics and a tour of Paris. There were 3 occasions in which my pack was notably tampered with when walking through crowded areas. The zipper lock prevented the bag from being opened and items from being stolen. With this bag I felt pretty comfortable walking around an not having items stolen. My friend that was traveling with was not as lucky, the bag that he had was opened when we passed through a crowded area in Paris.