I'm surprised people react so shocked. I asked if it could be anything other than an MMO because, well, it can't - it's definitely going to be some form of persistent universe. I'm also surprised at how many people worry about Bungie getting a good deal, or being incapable of setting up a good contract. They're in the perfect position to do this, and when they left Microsoft they made it very clear what they were after. You can bet they got pretty much exactly that.
Also, when Bungie says Activision, they mean the company that also is Blizzard, you know, those guys that run WoW. They'll work with that team to get the MMO infrastructure up and running - in fact I bet that they already talked with Blizzard and set up a good relationship before they signed the deal (which they mention was in the making over 9 months). The business infrastructure of Activision will be the global marketing and publishing, and support from its various other teams on the multi-platform development front.
There are just two things I think that can go south in a deal like this:
1. Activision will get a lot less from publishing Bungie's new IP than from a studio they actually own, and they'll mess up the marketing, lower the marketing budget, or mess up the release schedule. But I'm willing to bet that Bungie has negotiated that the release date will be of their own choosing, and as the marketing budget will probably be set to a specified minimum as well, and Activision will be wanting to make some money from this, it's unlikely that they'll mess that up. The actual marketing message could suck, but I'm sure Bungie has a veto or a creative say even here.
2. Activision will block Bungie from working together with their studios or with Blizzard, telling them to be as uncooperative as possible (probably even this is negotiated in the contract). But no part of Activision has that kind of say about Blizzard, and Blizzard doesn't strike me as the kind of team that would cooperate with anything like that.
Anyway, here's a good interview by Eurogamer. I think it's a must read if this interests you.
http://www.eurogamer.net/articles/bungie-explains-activision-deal-interview