yeah, that's not exactly an uncommon looking backpack. well, if nothing else, I'm sure the FBI has access to the same info, and will follow up on it as they see fit. I imagine the amount of video and photographs of the event is staggering.
I thought I heard something the other day that in this situation it would be almost impossible for some piece of evidence to not exist showing who planted the bomb, due to the massive amount of cameras that get trained on the route during the marathon. I think its just a matter of time and man power to sift through all of it.