The source of the "Gearbox if developing Halo 4" rumor we reported earlier in the week seems to be an email, supposedly sent from Director of Technology at Gearbox Corrinne Yu, that reads in part:
I was offered and recruited to the position of Principle (Lead) Engine Architect of Halo Franchise Team at Microsoft. I accepted the offer to lead the team to build the engines and games for future Halo's on Xbox'es and PC's at Microsoft.
While this doesn't exactly confirm that Gearbox is making the next Halo, it may point to just such an arrangement. We'll let you know when we know.