Cas is right. Its cheaper than the ship to shore phone calls BUT it is by no means as cheap as using it at home. You would have to look up your cell carrier's rates but most of the major cell carriers charge an international roaming rate for use on board. If you look here
you can see which ships (on all lines) that it is offered on and you can look under "roaming Partners" to see if your cellular provider is included and their rates.
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