In an old Interview, which I can not find atm, Todd said that when they are finishing one game (Oblivion in this case) they are in pre-production for another one (meaning Fallout:3 at the time) thus once one game is done using writers, concept artists, and such in one game they move right on to the next one with those doing the coding and such moving right in as soon as they have completed their part of the last game. So by the time testing is being done they would have the majority of the team working on the next game and the rest working with testers to work on tweaking, polishing and such.
That would mean that Skyrim was in pre-preduction for quite a while prior to the release of Fallout:3 with the entire team moving on to Skyrim upon release (minus the ones assigned to doing patching). Sooo, for two full years the entire team has been on Skyrim and more than likely many are already working on the next big game. Bethesda has had experience now both meeting release dates and missing them and pushing things back. By now they have a good idea of where they need to be prior to announcing a release date. I imagine this one will be released on time as planned. And since Todd said he was ready to show the game whenever Pete gave the go ahead back in Aug. and another 4 months have passed since then I would bet this game is very far along.
http://www.eurogamer.net/articles/2010-08-16-todd-howard-working-on-two-new-games
Howard said that the studio's current title will be announced soon, but he can't say exactly when. "I have a sense but we're not ready even to talk about [the timing of the announcement], because it might change. I don't want to disappoint people.
"One thing I can say is that from when you first hear about it to when it's out will be the shortest it's been for us. It's pretty far along. When we show it, we want to show a lot, because there's a lot of game there to play right now.
"You know, if [global VP of PR and marketing] Pete Hines came in and said, 'I want you to show it,' I'd be like, 'Okay, I'm ready to show it.' But we've just decided for now not to yet."