I have just finished the Dead Money DLC and have returned to the Mojave.
When starting the DLC a message told me that my companions had returned to wherever they came from. But when I go to recruit Arcade Gannnon, I get the response that I already have too many companions, when I don't have any.
I am playing the game on Xbox 360.

Just solved this. I went to recruit Rex first and then back to Gannon. He know lets me recruit him.
What a strange bug.