I loved FO 3 and a enjoyed FO:NV even more, and with the last DLC coming, marking the end for FO:NV im more and more starting to wonder whats next for the franschise. Unless obsidian is makeing more DLC, or bethesda have magic hands we wont see another fallout related game for what? 2-3 years, makes me sad :/
Probably a year or two until Fallout 4. Fallout 3 was in pre-production when Oblivion came out, IIRC, and there was a comment a while back about Skyrim that mentioned they had another of their franchises in pre-production right now, so its not hard to imagine it being Fallout. Although, of course, it could be something else.
Besides, I think Fallout: Between Good and Evil is coming out in the next few months, so that should keep people happy for a while.