If a football player catches the ball as he flies out of bounds but touches the end zone pylon is it incomplete or is it a touchdown?
It would only be considered a touchdown if the player catching the ball is deemed to be 'in control' of the ball as it crosses (touches) the pylon. Also, in the NFL, the player must have two feet firmly established inbounds before going out of bounds, while in NCAAF, the player only needs to have one foot established inbounds. If the player doesn't get the appropriate amount of feet in bounds AND establish (what the referees believe to be) control of the ball, then a touchdown is generally not awarded.
You can read more about "Completing the Catch" here on the NFL Operations website.