In baseball, what is the ruling when someone hits a ball foul but it then goes fair before it passes third or first base?

If a batted ball touches in foul territory but returns to fair territory before crossing first or third base AND is not touched by the team in the field while in foul territory, it is a fair ball. Watch the 1st or 3rd baseman next time you watch a baesbal game...they charge on a foul grounder to get it in foul teritory if it looks as though it may roll fair....

As a former umpire, I would support jbean's answer. Any ball which moves from foul to fair territory of its own accord before passing the base is a fair ball, if it stops in fair territory, or proceeds past the base in fair territory, or is then touched in fair territory.