I used to know a guy who did street photography and caught amazingly intimate shots of total strangers. I always asked him "How the hell did you capture that?" as I couldn't believe how close he got to these people, and how unaware they seemed of the camera. He said his best shots came from using a compact camera that he held against his chest; on the street or the subway, no one realized he was even shooting. But doing that with an SLR, he told me, would be out of the question.
It's for that reason that PhotoJojo is producing their Super-Secret Spy Lens. You simply screw it onto the end of your regular lens, enabling you to now shoot 90 degrees away from what your camera is pointed at; the SSS Lens isn't a lens at all, but a simple housing for a mirror.
As with my chest-shooting buddy, anyone paying close attention may realize you're shooting them; but this will surely increase your chances of getting that impossibly-candid shot.