Eurogamer has word from Pete Hines of Bethesda Softworks, who says a demo for Fallout 3 is not in the cards:

When you build it as one thing, there's no way to portion off a section and have it stand on its own without putting the whole game in the demo, which we're just not going to do. And it doesn't really capture the fun of a game like an Elder Scrolls or a Fallout, where you can go where you want and do what you want. So no demo, sorry.