I wouldn't be surprised if they are starting development on the sequel. You have to figure it will take (hopefully) 1.5 - 2 years for the sequel to come out, not be an annual launch like CoD or Assassin's Creed. Looking at Bungie's track record, development of the following Halo sequels took 2-3 years, I just don't know if Activision will pressure them to release it sooner, hopefully not the case.
We all knew a sequel was going to be in the works, I just hope it doesn't mean they abandon support for the current release, incoming DLC, and eventually get around to fixing the issues that have been plaguing the game. Maybe then they can work to improve or give the fans what they really want... you know, a pilgrim hat from the Postmaster for Thanksgiving or something.
Although disappointed from the hype, I am still enjoying the game despite its flaws. Although I've only partied with about half of the clan, I look forward to raid nights where the teamwork really shines through.