According to GameSpot's rumour section, the new Grand Theft Auto IV downloadable content will be called "Blood and a Four Leaf Clover" and will be released this Christmas.

QUOTE("GameSpot.com")
The second Xbox 360-exclusive downloadable episode for Grand Theft Auto IV, subtitled "Blood and a Four Leaf Clover." The title may be a reference to the GTAIV mission Three Leaf Clover,where protagonist Niko Bellic helps Irish mobster Packie McReary and his brothers pull off a bank robbery straight out of the film Heat. It's apparently due "this Christmas."

I personally find it rather strange how they stumbled upon this information, so don't take it as anything but a rumour at the moment, however it's entirely possible that information on the new DLC will be released at E3, so stay tuned for further updates.

Sources
GameSpot Rumours
Original Topic by ViceMan