It was wearing a case that made it look like a 3GS (in a case), though. You'd have to pick it up and stare at it to realize it had an extra camera cutout, etc. You'd have to be planning to do something with the phone already (steal it, or just return it to its owner) to take that long/hard a look.
It looked different. iPhones only come in a couple designs, it's not hard to tell when it's different.