The Official Fallout 3 Website has been updated with a teaser video. You can download the teaser in HD from WorthPlaying, FileShack or FilePlanet.
Said teaser uses game assets, no indication of wither F3 will be third person or first person perspective game, but by the very music used in the teaser it is clear that Bethesda knows the atmosphere that the series has. The teaser suggests a Fall 2008 release, but I think we can all count on F3 being delayed.