It's titled "LEGO Batman: The Movie - DC Super Heroes United".
Source: NYCC: Another LEGO Batman Movie in the Works!
Forget Batman: The Dark Knight Returns, Part 2, Superman Unbound, or Justice League Flashpoint, DC/WB gave out the first details of their upcoming movie LEGO Batman: The Movie - DC Super Heroes United!
Their New York Comic Con panel, which offered up information on 2013's animated home movie offerrings, kicked off with details and a trailer (sadly not available online yet) of this upcoming LEGO project.
The new movie will have the Joker and Lex Luthor teaming up to take Gotham apart "brick by brick". There will be appearances by Batman, Robin, Superman, Wonder Woman, Martian Manhunter, and Green Lantern Hal Jordan there to stop them. Joker and Luthor will not be alone though, as there will be appearances by the Penguin, Two-Face, and Harley Quinn.
Will Arnett will be the voice of LEGO Batman.
UPDATE: Trailer now available