I'm pretty positive that they were working on Fallout after Oblivion. The next Elder Scrolls might have had some planning during that time, but I don't see it having actually been in development. After Fallouts release I would think that they sort of split up their team, half to work on the DLC's and the other half for TES planning, and probably working on the engine.
Even half of their team, or hell, even a quarter of their team, working on TES V still constitutes development. I'm really doubtful that they were in the planning stages of TES V for the more than 2 years it took Fallout to be released, they must have moved onto development at some point before then. Plus, Hulu wasn't around in 2006.
On a serious note, when do you think we can expect Fallout 4, in relation to TES V's release?
My own idea of when it will come out comes from how long TES V takes to come out. It's been 4 1/2 years so far, but chances are good that even if it was announced right now by the time it was released it would have taken possibly 6 years. So, adding 6 years on to 2008 leads to late 2014, which sounds about right. :shrug: