Apparently it's going to be from the point of view of one of the Joker's new henchmen after Joker escapes from Arkham Asylum only to find the city being divvied up by several other denizens of Gotham, notably The Penguin, The Riddler, and Two Face. It was originally to be title Joker: The Dark Knight but for obvious reasons (namely the motion picture The Dark Knight) it was shortened to just "Joker".
The art work looks like it's going to be fantastic: the character design of the Joker is oddly similar to Heathe Ledger's portrayal in the movie, leaning away from his typical gaunt look. In fact, here are few pics to tide you guys over until the release:



Like I said, it looks pretty promising. We can only hope it turns out to be as iconic as the Killing Joke or Arkham Asylum: A Serious House on a Seirous Earth. Head on over to Newsarama for an interview with the writer.
~Luna