We renewed our vows on the Magic during the 10 night holiday cruise in December of 2004. We wanted it on the 7th deck but were told that it wasn't an option. Had to have it in Sessions (Cadillac Lounge on Wonder). Also, we were unable to pick the date - it's completely up to the Captain we were told. Ours was at 4:00 on the last sea day. I wasn't thrilled with having it inside, but it turned out beautiful. We met with concierage soon after boarding and she helped set up any excursions, spa appts., etc. Then we met with her the next day to finalize flowers, cake, music choices and photos. She "surprised" us every day with some sort of treat in our room. (We did not have concierage except through the vow renewal package, but were treated to all the concierage pluses.) If you have any specific questions, feel free to email me and I'll try to answer.
All in all, it was a wonderful experience and we're very happy that we did it.