In an old interview that I don't feel like finding, and the recent OXM interview, which is on the first page and don't feel like lining.
Are we talking interviews like http://spong.com/feature/10109908/Q-A-Bethesdas-Pete-Hines
No plans for a Shivering Isles expansion. We just felt like it was better to focus on the kind of DLC we've been doing as opposed to something that takes so long to make and comes out a year later, like Shivering Isles did. This is the last of the three we had announced we wanted to do, no announced plans beyond that.
Again, there's no explicit statement that they're never going to do expansions ever again, just that it wasn't in the cards for FO3. It can be implied that they're going to be heavily invested in DLC at the expense of expansions, but still, nowhere do they say, "no more expansions."
Regarding the interview, he never says anything explicit either. He talks about value, balancing cost and size and development time, but never does he say, "We're not doing expansions." In fact he says, "... what that formula will be down the road, I can't say today."
Can we again infer that DLC will be predominant, even all-encompassing? Sure. But there's still the possibility.