Beating a way dead horse here, but the same "what do I wear conversation" came up at QuakeCon yesterday. We thought a Vault 68 costume would be awesome (if you aren't familiar, the population of 68 was 999 women and 1 man. ) Basically the normal vault suit torn up, covered in lipstick, and smelling of perfume. As a bonus, you could get a few ladies in the 68 suit with lead pipes, black eyes, etc.
Kind of an uberspecific costume, but we thought it would be funny.