A little background for those not familiar with who 343 Industries is: when Bungie left Microsoft's exclusivity realm, Microsoft was forced with the task of finding a development studio that could properly handle the Halo
franchise. Instead of finding another development studio, they created their own. 343 Industries is an internal studio that is tasked with the specific purpose of handling the Halo
IP. The studio is currently helmed with the franchise steward Frank O'Connor. O'Connor was Bungie's content manager and the primary writer for the Bungie Weekly Updates during the development cycles of Halo 2 and Halo 3.
Follow so far? Excellent! How does this relate to the Master Chief? A quick refresher on the ending of Halo 3 (this is ending you received if you beat the game on Legendary)
You see, Master Chief's story was never properly concluded. Heading back into cyrogenic sleep. Leaving instructions with Cortana, "wake me when you need me," echoes loud within the halls of the Halo universe. O'Connor has confirmed during Comic-Con that what players see at the end of the Legendary ending is in fact a planet. He would provide no further details on the planet itself.
During the same event (via IGN), O'Connor provided this little insight on the above ending:
"We'd have to be the world's biggest a**holes not to follow through. We certainly haven't seen the last of Master Chief."
It is highly doubtful we will hear anything about Master Chief until after Halo: Reach releases in September. It is still nice to hear that our favorite bionically augmented military hero (sorry Ladd Spencer) is not forgotten.