It would have been a turnover of OM recovered and the ball stays live until it touches out of bounds or touches a player who is touching out of bounds.
It's not a fumble (it's treated like a fumble, though). Technically, the moment Henry released the ball, it becomes a loose ball play, by rule. When someone recovers it, the play becomes a running play (these are important for rules administration purposes). The difference between a forward and backward pass is that a forward pass becomes dead the moment it touches the ground, a backward pass does not.
Also, you can have multiple backward passes in a single down and receiver eligibility is not in question like on a forward pass (there are rules about when and how lineman can receive a backward pass, but they can, by rule).
So what the officials ruled on that fourth down play was whether it was a backward pass or a fumble, which are technically two different things in the rulebook.
