I can confirm that paid DLC will consist of all new maps, missions etc.
GIHAINIDII is IW's info person on the forums (his/her profile says IW Developer), so you can trust this. I'm mildly disappointed, there are some old maps that I really, really liked, but I understand it from a PR perspective. It's still possible that old maps will come out (notice it says "paid DLC") but I think it's highly unlikely.
Pointer from MP1st.
There are old map files in MW3 though. CharlieIntel just posted about Terminal being there. I'm curious how this will play out, especially since the map file is listed as a Spec Ops mission.
Also, in the MW3 forums GIHAINIDII responded to a question about old maps: