I might actually get a shirt with something programming-associated on it. Not like one of those "I CODE LISP" or "RUBY ON RAILS 4 LYFE", because those sort of shirts advertise that you are a nerdy moron with stupid technical and esoteric jokes.
A better one would be simply to have:
--___--
/ \
--' λ '--
| |
|_____|
Because then it just looks like one of these stupid cheapo brand logos/designs that you get on shirts. No normal person will question it or notice it. Hell, it will be invisible.
Whereas someone who knows what lambda is may approach you and start a conversation, thus increasing your chances of meeting new people with similar interests.
So on one hand you're advertising that you're a nerd, but in a subtle, non-obnoxious way, to other nerds, but looking like a normal guy to everyone else.