On all the cruiselines we have gone on, the rule has been...you are allowed to bring 1 bottle of wine/champagne/pp, reasonable amount of water and soda but no alcohol. If it is brought on board, it is ususally smuggled on in various ways or just packed securely, overlooked and the cruiser was lucky.
Teresa and Larry
#36-Carnival Splendor 9/16/12
7 night Mexican Riviera
#37-Allure of the Seas 11/11/12
7 night Eastern Caribbean
there is no cruise-wide policy it varies from line to line.
Most cruise lines will allow you bring aboard a reasonable amount of champagne or wine. Other alcoholic beverages are not officially allowed, but as you have read, many people will smuggle it aboard in their checked baggage. If you pack it in your carry-on you increase the risk of it being taken from you at check in.(all depends upon security on this point)
FYI - the average price for a good quality bar drink on the ship is about the same as you would pay back home at a good restaurant.
very easy to sneak in. parents sneak liquor in the plastic bottles every cruise. i made it on with a case of budlight last time. to cut down on t he first 2 days of beer or when its late at night and they are out. trust me you cna get away with it.