Ausir at the Vault has penned up an article where he details most of the speculation that's been circulating on the various Fallout fansites (including this and the Vault itself) for a bit. The arguments are pretty solid, so if you're interested in what the new DLCs may be give it a read.
Dead Money was first hinted via in-game graffiti. The textures for these graffiti are in a folder in the game files called "nvdlcgraffiti", and all the textures share the filename "nvgraffitisierra". Interestingly, this folder contains a number of other textures, sharing the filename "nvgraffitihonesthearts". These read:
"Where's New Canaan, anyway?"
"The Burned Man Walks!"
"Joshua Graham Lives!"
This implies that one of the future add-ons will be titled Honest Hearts and will feature Joshua Graham, Caesar's former right hand, who is also known as the Burned Man, as he was set on fire and thrown into the Grand Canyon for his defeat during the First Battle of Hoover Dam