The English Hallowe'en tradition is that kids dress up as something scary: a ghost or a vampire or a zombie or a monster. In the US the tradition is that everyone dresses up as anything. It's a like a fancy-dress free-for-all: a car air freshener, the fail whale, a chick magnet... anything at all, it seems. Trick-or-treating is roughly the same both sides of the Atlantic, although the solid expectation over here is treat rather than trick.
On Sunday we went to a Hallowe'en party in Marin. Wendy and I intended to dress up but didn't quite make it (our costumes stayed in the car). Lux, though, went as a woolly lamb. The hood of her top had ears on it, but you can't see them in the best shot I have. I'm not sure what she made of the whole thing.