Old World Blues and Lonesome Road.
not much info about either of them known yet.
[Thanks, i think there's some information floating around on the Facebook 'Fallout New vegas' groups and some information floating around here about i think its old world blues? I dont know].
Old World Blues: you get trapped in the big empty which is an area run by robots and computers. You have to find pre-war technology to either defeat your kidnappers or help them in their goals against a bigger threat.
Lonesome Road: you get a broadcast message from ulysses and journey to the lonesome road an area feared by many because of its vastness and constant storms. You search for ulysses for answers and a final confrontation.
The rest two DLCs will all come out until the end of this year. unconfirmed information says, Old World Blues comes out in June, and Lonesome Road in July, but it is not certain.