I'm from the UK, but if a guy went up to me and put his arm around me while talking to me then that's what I consider it to be, too. There are a couple of guys that try to do this sometimes, and I've never quite been sure what the correct response is.
Stop moving and stare them in the eye with a vaguely irritated look.
Far different from a light touch on the elbow and "can I speak to you a second" or "you look familiar, have I seen you at _x_" but point taken.
T-shirt idea:
For your own safety . . .
(Back)
. . . don't touch anything on this side without permission.
(Lower back)
And if you're reading this and not a kid, pick yourself up and reread the above. Have a nice day.
http://www.youtube.com/watch?v=KNKNAK60AXk&t=1m10s