Thanks bluezebra,
I wasn't claiming to be an umpire in the know, just guessing what the answer was, so I'm not feeling slammed. It seems I was partially right (not foul until it hits foul) and the possibility of it returning to fair ground, however unlikely, justifies running. I didn't know about the F1/F2 etc. scenario (since I'm not an ump and don't claim to be one), so I learned something today.
The "Uncatchable" issue should cover the situation, but apparently the ump of the moment didn't think it was uncatchable. Wouldn't that make it a judgement call and thus, not protestable/arguable?
Also, how do you determine who is the "first fielder" thus affording the protection to him?