This not based on first hand knowledge, but I remember reading something on a message board about a past cruise on any of the lines owned by Carnival (like Seabourn is) will get you a small "past passenger" discount or something of the sort on their other lines. Again, not sure of the particulars. At the time though, I thought, "Oh, that would be good. I took a Carnival cruise years ago!"

I would love to go on a free cruise on the new Seabourn ship based on my nights with Regent. That's about as sure a thing as proverbial pigs flying, though!