I just found this and I am totally doing this for next year! I think it would be fun if you had to have each person you find sign the card- although a couple of them might be a little awkward :) If I find a hi-res version I will post it too.
Crazy 4 Comic-Con
Tips on the San Diego Comic-Con and other conventions
Ha! Its brilliant! I found at least 15 of them at this years Con