Yup, my experience has been the same as Billy's. If you get there early (11ish) you can most times get on a little earlier than the posted 1PM.
We got at the port at 10:30AM for this last cruise (because that is when the hotel shuttle could get us there) and we stood in line for about 45 mins and were on the ship at 11:30!! They even handed out a sheet of paper explaining that though they normally don't start boarding until 1PM, they were ahead of schedule and could offer us early boarding. Just seems like you get a bit more "cruising" time than if you wait until 3 to board. I also figured I could wait there at the ship and maube get on early or wait at the hotel and gaurantee I don't.
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, Celebrity Silhouette 2013