taking pictures without being noticed takes a bit of skill, but surprisingly little -- ditch the digital slr and find a small point and shoot -- Olympus XA, leica CL, something small and black and inconspicuous to start with.
pre-focus, learn to shoot from the hip, even be looking in a different direction while you shoot.
i've had this happen often -- I will snap a coupla shots of a homeless person, begger or something of that sort, and then the person will notice the camera, at that point smile, say hi, ask if you can shoot them, give them a buck, take a couple of posed shots (everyone loves to post) and then when they are tired of your game they will go back about their business and you can shoot them without them noticing again, but this time with their permission.
A rolleiflex, by the way, makes a GREAT camera for this -- everyone is trained to look for big honking digital cameras, or telephones, or something. Nobody is looking for this black thing hanging at waist level that you look down into a few times.
all about being something other than what people are used to seeing.