According to various sites including The Halo Council, 343 touched on the subject of a follow-up to Halo: Combat Evolved Anniversary in a recent Q&A posted on Microsoft ExpertZone, a registration-only resource the platform holder uses to train retail partners.
Asked if there was any chance we'll see Halo 2 HD before the end of this console generation, 343 reportedly said: "We are not working on that at this time. We are kind of busy."
An Xbox PR representative told us they couldn't confirm whether the supposed Microsoft ExpertZone Q&A was the real deal or not, but added: "Our focus right now is on developing Halo 4. Beyond that, we do not comment on rumours or speculation."