The editor, available for free, can be downloaded from Bethesda's site. Anyone who has followed Bethesda games in the past, such as Morrowind, can testify to how much extended life easily-moddable games receive. If the past is of any measure, Fallout 3 will soon have a host of user-generated content available. Bethesda still has expansion packs planned for the game, which are not due until next year.
Allowing people to edit and modify games is something that game developers have gone back and forth over for many years. Some companies have despised it, even seeking to stifle any 3rd-party additions to their games. Others have embraced it, and it is easy to argue that giving players that freedom breathes new life into games that might otherwise slip into obscurity.