Frank himself has said it has been played through. They have shown it to and allowed their marketing partners to play it as well. There is, as John mentioned, need to shake out some more bugs The MM is running as well. Here is a direct quote.
"Obviously we’ve been working on the game in earnest for a long time, but it’s all starting to feel pretty real to me now. Campaign is playable from start to finish (not to be confused with complete or polished) and multiplayer has been running for a while. It’s amazing to see changes big and small have a dramatic effect on making the game smoother, more playable and more enjoyable in almost every session.
Personally I have been traveling a fair amount in support of the impending year-long launch activity. Visiting with our partners and colleagues in Europe to talk (and walk) them through the game, the features, the U.S. marketing plans and the sheer scale of the project. We certainly surprised some folks with the ambition and concrete realization of where we’re at, but it was kind of cool to see genuine excitement and enthusiasm for the game.
Our show and tell was pretty BIG, since the European partners (marketing, retail guys, etc.) need as much information as is available to make their plans and start building the path to launch abroad. So they went from knowing practically nothing about the game, to knowing practically everything about it. Which is a weird feeling when you expend so much effort and energy on security and secrecy. It’s amazing how much detail you need to go into. It’s not good enough to simply list off the changes and additions and improvements, you have to contextualize them. It’s been almost five years since Halo 3 and a LOT of stuff has happened between then and now: ODST, Reach, multiple map packs, loads of story and a whole swathe of technical and gameplay changes.
Creating material to explain those changes, walking people through the game features – showing them a lot of this stuff in action, it’s fun, but it’s nerve-wracking. In a way it’s a preview of our public showings, where we get a feel for which changes, which additions, which refinements are going to get people excited. It’s also a chance to get a feel for which new features are complicated and how to distill the explanations to capture the essence of what we’re making. Training wheels, as it were, for showing you guys.
Launching a game is a massive undertaking, with literally hundreds of people all over the world preparing and planning to make sure that every aspect, from the box art to the matchmaking, is executed flawlessly. And if it seems slow, ponderous, even, then remember that there’s a plan and a process, and that you’ll see Halo 4 soon enough. And we can’t wait to share it with you."
Frank O'Connor
This can be read in the "Halo 4 Facts and Rumors" thread and is also in the "News" in the top menu bar.