Not really. You can order a full cake to come to the dinning table. Its an extra charge but I am not sure how much. There is also several gifts you could get for them through Carnival's Bon Voyage department (find that on their website).
We were celebrating our 20th anniversary on our last cruise and our wait staff took it upon themselves to create a Happy Anniversary cake from a couple of chocolate melting cakes and some birthday candles which was cute.. but no official gifts. If you mention it to the maitre'd he might make an announcement for them.
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