Gearbox Software is the developer of the first game and right now they are all hands on deck helping to make CoD MW3. My guess is assuming Activision doesn't buy them out and force them to make CoD games probably in 2yrs.
They just released probably the worst received game of this generation as well with Duke Nukem Forever. I know they want to revisit that franchise they have gone on record saying so, but I am not sure if the game is even past preproduction phase yet.
I am guessing if you see a trailer for it next E3, expect a 2013 release date.