Check out this funky spin:
As long as the ball has not crossed first or third base, or if a fly ball has not landed beyond first and third base, it is neither fair or foul until the ball is touched or comes to rest. This ball starts five feet foul and then comes back fair.
It is not touched until fair so this is a fair ball. Nice job by the umpires to wait until there is a call to be made. Oftentimes, you will see umpires just stare at an obvious foul ball and not do anything until it is touched (if at all).
This is an umpire who has mistakenly called foul too early in his past. Good work by the crew on this call.
Fair/foul is covered on page 37 of RuleGraphics.