Not sure if it's already been offered up here, but my first choice is Vincent Cassel. He's a great actor and could look the part for sure. He can play coy, devious, menacing, and physical as well. The french accent would be interesting too - would add a degree of suaveness to the Joker that could be refreshing.
I know this has been kicked around on the internets for awhile...not the look I'd ultimately go with, but it gives a good reference.