Oblivion and Fallout 3 used the same version of the engine. It sounds like TES V will used an improved version of the Oblivion/Fallout 3 engine.
And pre-production started at least a year before the engine was ready. They don't wait to start a game the day the tweaks for the engine have been finished. That year of pre-production is always counted as part of the 3-4 year total production.
With a late 2007 pre-production start date and TES V being their main focus since late 2008....makes a late 2010 release date plausible.
I really doubt they were in pre-production that long. I personally think that a 2010
announcement is possible, but a 2010 release date is very very unlikely. One bit of evidence is the fact that after Bethesda announces their games, they develop them for at least another year before releasing them. It is really impossible for it to be announced in 2010, because Bethesda didn't announce it in 2009.
Morrowind announcement- Morrowind release: 25 months
Oblivion announcement- Oblivion release: 18 months
Fallout 3 had a 4 year announcement/release period, but that doesn't count because it was unofficially "announced" in 2004 when the Fallout license was bought. Unless Bethesda magically changes their ways, we wont be seeing TESV in 2010.