There are a number of iphone apps for this kind of thing, like SneakyPix and NinjaCam, among many others. Do a search for something like "secret photo" and you'll find a bunch of them. That said, getting up the nerve to use one? There ain't an app for that--I sat across from someone on the train tonight who'd be perfect here, but at a range of 3 feet between us, no way was I going to try to snap a pic.