Technically yes, what Sandy says it true. But even at the front or the back, you are not going to "feel" much motion. If the seas are bad enough to feel motion you are probably going to feel it in the middle of the ship also. THink about it. The ship is riding on the water's surface, if the water is moving up an down, so will the ship. All of the ship. If you are the type to get sea sick then the little less movement you might have by being in the middle of the ship is not going to help that much.
Having said that, in my 14 cruises I have only been able to feel the ship moving twice. So I wouldn't let not being able to get a mid-ship cabin keep me from cruising.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007