In a new interview with Emil Pagliarulo, lead designer of Fallout 3, he confirms some new information on the game. The main game, minus all of the side-quests (which there are many of), is 30 hours long. It is very easy to get sidetracked though. He also describes some differences on the VATS system in Fallout 2 and Fallout 3. Watch the full interview above.
[Exclusive Fallout 3 Interview from PAX ‘08]