Well here's a treat for you - I don't think you can really consider this a spoiler, but Robert from FilmsPunk.blogspot.com alerted us to a test picture of Heath Ledger wearing his Joker prosthetics without the white. black and red makeup.
He said the image originated at the SuperHeroHype forums, but I wasn't able to find it over there to provide a proper credit.
You can check out the image below (which I've enhanced a bit to bring out the detail), plus we've got some interesting info regarding the makeup itself...
It seems that the person who came up with the makeup design for the Joker in The Dark Knight was... Heath Ledger himself! According to the poster over at the SHH forum:
"he [Heath] showed up with a very basic make-up kit consisting of store bought mascara, white grease paint and lipstick and and basically designed the make-up there on the spot"
Apparently everyone loved it and then the makeup artists had the task of recreating it every day for shooting.
Anyway, here's a look at Heath wearing only the prosthetics (which he did not design):
Pretty interesting story... for more details and the full quote from the source head over to FilmsPunk.blogspot.com and thanks again to Robert for the heads up!
The Dark Knight opens on July 18 (as if you didn't know).