Frankie (OuterWorldVoice on GAF) said this about the news:
I wouldn't get too excited. Very minor.
and
The fact that it's Conan related is sort of major to me as a proponent of TEAM COCO, but the news itself won't be world-shattering, just important.
Those quotes found here and here.
If I had to speculate, I'd guess a release date announcement plus a few details.
edit: Found this on Twitter, supports a release date announcement:
edit 2: credits go to Sean for finding this:
Polygon/The Verge spoiling the "surprise" - November 6th release date they say.
No other details are being offered, but we've heard from reliable sources close to Microsoft that the team may have finalized a release date for Master Chief's return: Tuesday, November 6th, 2012.
We've also heard that it'll be announced before noon ET tomorrow. Though development cycles tend to fluctuate, locking down a pre-Black Friday release date this early gives Microsoft's marketing team ample time to plan for what we'd suspect will be one its biggest game launch this year. Will Conan and the gang be announcing the launch date? We'll know soon enough.
http://www.theverge.com/2012/4/16/2953225/halo-4-conan-obrien-source-availability
edit 3:
Watch, laugh, enjoy. Entire Conan Halo 4 stuff http://www.youtube.com/watch?feature=player_embedded&v=GflJB-aM0wI#!