2 years right after Oblivion of fine tuning their engine, 2006 - 2008
Bethesda improved upon the Oblivion engine for Fallout 3, I'd guess they've either further improved this engine or started fresh with a later version of GameBryo - buth either way they wouldn't have started this until they were done working on the Fallout 3 engine.
Or maybe they pull a 'New Vegas' and tell Arkane to make a third adventures game, a year or two after TES V.
When Obsidian got the rights to make a Fallout game this fact had to be announced, I'd guess if Arkane got the rights to make an Elder Scrolls game they'd need to announce it as well. I suppose it's possible that Arkane didn't officially start this project until they had joined Zenimax but I bet they've got different under wraps right now. An Elder Scrolls spin-off may be a possibility for the future though!
On another note...has anyone considered that the game in pre-production is a new IP? Maybe Obsidian will take on the Fallout series and BGS will start a new IP for the next gen consoles.
Yes, this was discussed http://www.gamesas.com/index.php?/topic/1110982-official-tes-v-speculation-thread-61/page__view__findpost__p__16288701 in fact.
I myself am not adverse to a new IP from Bethesda but now just seems like very poor timing.
I seriously doubt Bethesda is going to give Obsidian the reigns to the Fallout IP. Bethesda has stated in the past they're interested interested in making more Fallout games so I don't see why they'd stop. Are future Fallout games from Obsidian also a possiblity? Certainly, but these will be in addition to Fallout games from Bethesda.