I enjoyed this fellow's sign, which on the other side said something like "I'm an Earthling too, so we probably have something in common." The side visible here is James W. Cadle’s Earth flag -- I don't recall seeing it before, but its symbolism is immediately obvious to the science geek. I'll be getting one soon. I also liked that the guy with this sign was wearing a Carl Sagan-esque brown corduroy jacket -- whether coincidence or part of a subtle Halloween outfit, I'm not sure.
I also like the "Not All Georgians Are Redneck Republicans" sign.