Well, I did my White Picket Fence costume, and not a single person--even my most incorrigible punster friends--got it without prompting. Once folks got it, they sometimes ran away in disgust, though, so it was all worthwhile. I think it's the most obscure costume I've done yet.
My wife, who dressed as a pirate wearing a bandolier covered in old computer game CDs and manuals, had an easier time; plenty of people realized she was a Software Pirate.
My two favorite reactions of the night:
1) The guy that saw my wife, nodded happily, and said, "Cool, a Canadian pirate!" We still have no idea what he was talking about, but it really amused us.
2) The trustafarian who read my sign "Workers Revolutionary Engarde Party--Our Movement Will Not Be Foiled!" and gave me a peace sign and said "Yeah! Viva la revolucion!" I was mortified at first that people thought I was trying to politicize Hallowe'en, until I realized exactly how stupid a response that was, and started giggling a lot.
Daniel