In an interview with SLTToday.com, Heather Ledger says that The Joker will definitely be an American in The Dark Knight:
For his vocal chords, it was a short-lived respite. Soon he’ll play the Joker in “The Dark Knight,” the new Batman movie.
“The Joker,” he says, “is definitely an American.”
The Batman Begins follow-up is scheduled for a July 18, 2008 release.