Tipster Jecrell not only gave us the above YouTube link to Bethesda's teaser trailer for Fallout 3, but he also directed us to Voodoo's blog, which has some extra info on it. Pete Hines, Bethesda's PR guy, says:

1) Yes, that's in-engine (in-engine means it's done with game assets in our rendering engine, but isn't done real-time. If it was done in real-time, that'd be gameplay footage. This isn't something, for example, that was sent to some house [e.g. Blue Studios] so they could build assets and render it out, and create something for us to release [e.g. Dawn of War 40K intro cinematic]).
2) Yes, that's The Ink Spots singing "I Don't Want to Set the World On Fire"
3) Yes, that's Ron Perlman [voiceover man extraordinaire of Fallout 1 and 2].

So in the end, it appears the Fallout 3 teaser really was just a teaser: despite all my wishing and hoping that it might shed some light on our most burning questions about Bethesda's first-ever Fallout title, it really only gives us a location (the east coast, as shown by the lead-up concept art screens and the Washington Monument visible in the background), a vague release date (Fall 2008), and the presence of an organization that everyone knew would be present in the game no matter what (The Brotherhood of Steel).

But what of the combat? Will it be realtime, or turn based? Will the game be first person, a la Oblivion, or stick with the top-down isometric view famous to the series? Can I recruit Dogmeat? The teaser (like all good teasers) answers none of these questions.