Okay we have to calm down...
1) TES IV Oblivion was in creation right after Morrowind (2002) and released in 2006. Four years later!
2) In 2004, Bethesda began to develop Fallout 3 (phase of preproduction until the release of Oblivion in 2006) and released it in 2008. Four years later!
3) In 2006, Zenimax has trademarked the name of Skyrim. So, if they knew the name and the place of there next big game at this moment, they knew more or less the story and the background of the game. So they have started to work on since 2006 (phase of preproduction since the release of Fallout 3 in 2008).
2010 = Four years later! Isn't it?
PS: I doubt they release TES V in 2010 but this is not impossible!
I think the preproduction is kinda unimportant, what matters is the actual fulltime development time, at least when it comes to release date speculation. It only been 1.5 years since Fallout 3 was released, not enough time to make a new large game, especially not as Todd hinted that they are redoing a lot of tech while Fallout 3 pretty much just used what Oblivion used.
So here's my list!
1) TES IV Oblivion was in creation right after Morrowind (june 2002) and released in march 2006, and incorperated a lot of new tech with Radiant AI, Havok, facegen, Speedtree and so on. Almost 4 years later.
2) Full production of Fallout 3 started in april 2006 and released it in october 2008, took only about 2.5 years later seeing it pretty much just re-use the Oblivion technology. I also wonder how much time they need to build a wasteland game world compared to a living fantasy game world, I would guess it takes less time as well.
3) Full production of game X started in november 2009, and as Todd Howard recently said, it will again have a lot of it's tech redone and only been worked on for slightly over a year. I think it's a good chance that we will have to wait another 2-2.5 years until the game is done, about a total amount of ~3.5 years of full development time.
So my guess is 2012, but of course I could be very wrong about that