Exactly. They could easily create concept art and write most of the lore without thinking about the engine. After all, somebody here mentioned that Todd said that he wanted the game to have a greater atmosphere.
Also, it is likely that bethesda is withholding an announcement on purpose. They probably feel ashamed about showing stuff at E3 and then cutting it from the final build of Oblivion. So this time they might announce Skyrim prior to E3 and show something close to the final build at E3. And they may very well release it this year. If the game is any good, it will be very easy for them to build hype in 3 months. They certainly won't release the game in the 1st quarter of 2011 because lots of amazing RPGs are already scheduled for release at that time. There is also Fable III coming out on Christmas but I don't think that's a problem since the two franchises are very different.
I think it'd be extremely unlikely for Bethesda to release the game only a few months after announcing it. Bethesda doesn't roll that way. They always show off their games for at least a year before releasing it, sometimes close to two years. Bethesda loves to build up hype slowly, they are not the kind of developer to release a game a few months after releasing it.
If they announce it at E3 in June, there's really no chance that they'll release it anytime before Q3 or Q4 2011, and even those dates are pretty optimistic.
Well... I don't know how Bethesda works specifically. But recently team shifting has gotten all sorts of weird. I.E. the guys working on SC2 initially went over to finish Wow (yes, they started that long ago) and after Wow came out they restarted the game. The Valve guys updating TF2 all switched too finishing Left 4 Dead 2 a few months before it went gold.
As for Bethesda specifically...
They're making Oblivion and Fallout 3
Oblivion get's released. Oblivion team does?
Shiver Isles and Oblivion dlc finished, those guys go do?
Fallout 3 gets finished. Fallout 3 guys go to do?
Fallout DLC finishes, Those guys go to do?
Lots of employees and proof they do work on multiple big things at once. Heck before Dragon Age even finished some guys were already working on Dragon Age 2. Kinda svcks that they don't get at least a little break : ( but what this means for Bethesda is that it's obviously possible the game has been in production for quite a while.
Oblivion get's released. Oblivion team does?------------- Work on Fallout 3
Shiver Isles and Oblivion dlc finished, those guys go do?------------- Work on Fallout 3
Fallout 3 gets finished. Fallout 3 guys go to do?------------- Work on TESV and Fallout 3 DLC
Fallout DLC finishes, Those guys go to do?------------- Work on TESV
Bioware, Blizzard and Valve are all much bigger than Bethesda. Bethesda can't split up it's team into more than two or three teams at a time as they don't have enough people.