Depends on what you call the "Midnight Buffet". There is one Gala Buffet on a 7 day cruise. This is the grand buffet that they go all out for. They will open the doors an hour early for picture taking and then again for eating later. Aside from the Gala buffet and depending on the cruise, they will also have late night buffet of various themes through out the week. - Mexican food buffet, italian buffet. On the Conquest, they had a Chocolate Buffet (in the middle of the day, not at night) that is to die for. You never knew that so many things could be made from chocolate.
There really is food 24/7 on a cruise ship nowdays. The midnight buffets have become more for the entertainment value than an actual meal. On the Conquest, there is the grill and pizza open 24hrs and there are buffet lines open most of the time and there are also Chinese buffets and a deli and a fish and chips resturant. (You need to try a deli sandwich and a fish and chips at least once during your cruise.)
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