Just returned from the 5 Day New York - Maritime Canada itinerary aboard the Victory (7/26/03). I've never cruised Carnival before but nonetheless received an invitation under my door to the Past Guests Party. It was about an hour long, and was held on our last "Fun Day at Sea," so we decided to go. It was a pretty sedate affair, but honestly, it was worth it just to go for an hour and suck down the free booze. ;-) They only required that we dress in "Smart Casual Attire."
I'd definitely do it again, and was pleasantly surprised to receive the invitation!