I realize that I am bringing this back from the dead, but Im new here and I had one thought that I didnt see anyone mention.
Toe-tap vs. Heel-Toe:
In a situation where the receiver is laying out, falling forward OOB and gets a toe down (or foot + toe drag in NFL), no part of the foot ever touches OOB...100% of the contact (albeit just a toe) in inbounds. Typically the first thing to touch OOB is an arm or shoulder as they go to the ground, or they lift their foot to make another step and try and stay upright. If it is just a toe, and any of the toe is on white, you would call incomplete.
Contrast that with a receiver falling backwards with a catch. The physics of his body allow him to get a toe down, but in falling backwards, if the heel touches white, then it is still the same "foot" and should be
I think to call this complete, the receiver would have to get the toe down, and lift the foot to make another step and prevent the heel from touching OOB.
In either case, the Shipley catch would be tough for even the most veteran officials to call accurately in real time.