Oh but come now, 'a while' rings about two to three years in my reckoning. We will see, I suppose.
'A while' could mean anything from a month to a hundred years. We shouldn't use that for a basis.
I've got a theory. Let's look at the numbers, shall we? Morrowind was released on May 1st, 2002. Four years later, on march 20th 2006, Oblivion was released. Two and a half years after that Fallout 3 was released. The time between Oblivion and FO3 is only two years (as opposed to Morrowind-Oblivion's four years) because they had a seperate team working on FO3 as soon as they aqcuired the rights. Whereas before, when gamesas ws a much smaller company, everyone worked on one game at a time. Now that they've expended they can have at least one small portion of the team working on a seperate game while another game is polished up. In this case, while Oblivion was being developed, FO3 was at least being conceptualized. And while Morrowind was being finished, a small team was planning out Oblivion. I believe they've said this in interviews. So a conclusion can be reached that they are working on TESV at this very moment (obviously), and have been ever since FO3 was in developement.
Now, with my marketing experience, and studying gamesas's buisiness model, this is what I predict.
The first TES novel will be released this November. Perhaps TESV will be officially announced along side it, thus bringing attention to the book and at the same time generating hype for the fifth game. Maybe we'll also get a single screenshot too just to wet our apetites, kind of like what they did with Oblivion's announcement with the screenshot of the guy in steel armor on a horse looking out at some poorly rendered mountains. Whew.
Then we don't here anything from bethesda for months. Maybe they do some interviews, but they're tight-lipped about game features and what have you. E3 2010 rolls around and thats when they finally showcase their workable demo to the massess. We see how awesome it is. From there, they slowly release more screenshots, videos, and game mechanics up until a 2011 Q2 game release. They build us up, creating more hype like all developers do.
That's three years development between Fallout and TESV, more than enough time to make a good game, especialy if we take in the idea that concepts for TESV have probably been around ever since Oblivion was released, and that a small team was devoted to figuring out the basics. Now that Fallout3 has been out for a year, it's likely the entire team is working full-time on the next TES game. If they wait too long (someone a few posts back mentioned four years from now was likely), then people loose interest and the fanbase dissipates. Too soon and the game comes out sloppy. You have to aim for that sweet spot in the middle.
Remember Q2 2011. You saw it here first.