I too have thought about that shirt for a long time, although I figure it'll probably be bothersome to get it sent over the atlantic. I don't know if people in general will be more likely to approach you, but certain kinds of people might. Also I think those who do might feel more comfortable with you, knowing that you're you're just being shy, not aggressive (a regular misconception due to a lack of Theory of Autistic Mind).
It could also be a good ice-breaker, both because it's a great shirt, and because xkcd is really popular amongst certain kinds of people.