Very illuminating discussion guys... having been one of those holding the opposite interpretation for years, I have to say that I'm persuaded that the "airborne v. running" distinction is the key under the text of the rules and AR's. The interpretation also is in keeping with the underlying forward progress assumptions that must have been in place in the old TD AR's involving goal line extension, as well as this year's goal line extension "exceptions" in the TD AR's.
Don't know now whether to flag with that defender who, in an effort to stop forward progress, hits the runner over OB territory whose back foot has just left the ground inbounds and before his front foot hits the ground OB, but I guess that's for another day.
I'll go back to my lurking...