i imagine that it will be a subscription based game almost certainly, with some possibility for microtransactions on top of a monthly.
an MMO using a license as valuable as Fallout needs to generate a big return on its already substantial investment. not to mention, there has been some big expenses for gamesas as far as getting the game to where it is today -- legal, etc. monetizing the popularity of the license is important... they want to stay in the business of making games
and as a fan who has played most of the gamesas games (Castles was my first), I'm all for supporting them with a monthly subscription.