sub movement in a ported box

The same.  The movement of the cone is what produces SPL, and in a ported box the SPL doesn't increase at any point but at the point centered at the tuning frequency.  That's only because the box itself is producing sound at that point.  "Free air" is a misnomer when you are talking about cone movement in a vented enclosure.  The volume of air in the port and in the box itself is mass and controls the cone movement just as it is controlled in a sealed enclosure.  There's not that big of a difference between the two.  The difference is only the enclosure itself,  where a ported makes its own sound to complement (or should I say, add to) the sound of the driver.

This, like just about anything I say, is just a knee-jerk opinion based on what I've learned. 

Well put stevdart

Stevdart and Ispker:  well put.

Because I like adding cites so that searchers can get a complete story, here's Dickason:

"At low frequencies, the vent contributes substantially to the sound output of the system.  It does so, however, by increasing the acoustic load at the rear of the cone, reducing cone motion and the output of the driver."  Vance Dickason, The Loudspeaker Design Cookbook, 6th ed., Oct. 2000.