You'd lose the most essential aspect of the game, which is choice. The player writes his/her own story. That's the core of F3, and a movie simply could not capture it.
That's where people make their mistake. we are not talking about Fallout the game, but Fallout the movie, based on Fallout the game There would need to be certain aspects of the game altered somewhat in order for it to work a little better in a movie. That has been done time and time again when adapting books for a movie. There is no possible way to include every aspect of the game into a Fallout movie, which is why I said that it would be necessary to choose which parts to include and which parts should be omitted, or left for a sequel if successful enough.
You can still omit parts of the game from the movie and still get the point of it across. Also, there is nothing that says anywhere that a movie that is based on a game has to share that same name. Example: Post Apocalyptia: A Film based on the video game Fallout 3. Not the best example, but it gets the point across.
Of course the die hard fans of the game probably would find faults with it no matter what, just the same as the hard fans of the original fallout games find fault with Fallout 3. Same thing with the die hard fans of Morrowind finding fault with Oblivion, you just won't be able to please these people. IMO That isn't really the type of audience you really need to attract to the movie. Maybe, just maybe someone who watches the movie and hasn't played the game may say, hey maybe i will try that out, it looks like it might be good.