Can anyone give the skinny on how to best cut a shadow-box styled fence with a very gradual radius. PVC pipe? Thanks

 

If I understand correctly you want a gentle arc cut across the top of a section of fence. If this is correct I would measure the section of fence and find the middle, put a nail in it with a piece of string tied to it with a pencil tied to the other end make the string long enough to reach both sides of the fence as well as the top center and then starting in the middle strike the arc on both sides of the center. Play around until you are happy with the outcome and cut it off. The closer to the top of the fence you put the nail the more gradual the arc, the further from the top you get a steeper arc.
Hope this is what you were after.