They start boarding earlier. Most ports you can start checking in around 11 or 12.
Carnival Dream for Thanksgiving
Eastern Caribbean- 11/20/2010
Carnival Ecstasy (4 Day Baja Mexico) 2003
Carnival Pride (7 Day Mexican Riviera) 2007
Carnival Spirit (8 Day Exotic Mexican Riviera) 2007
Carnival Freedom (7 Day Western Caribbean) 2008
Carnival Elation (4 Day Baja Mexico) 2009
Carnival Spirit (7 Day Southbound Alaska) 2009
While many of the cruise ships will let you on earlier than the "official" embarkation time, they may not let you go to your cabin. Don't worry---they will feed you all afternoon until your cabin is ready.
I just got back from the Estascy and I was on by 11.45 or 12.30 and on the Lido deck eating. There was NO line to check in. We had to sit in the chairs for about 30 minutes if that long and then we were allowed to board. I had to wait until 1.30 to get to my cabin, but no big deal.