My husband & I were on Z in October, we had the same nights as Pete. There were some that did not dress on formal nights, but they were in the minority. We kept our formal clothes on through the evening (show, clubs, etc.) it was just more fun! What I thought would be a problem for my husband (dressing formally on vacation) pleasantly turned into him enjoying himself immensely. The night for us started at dinner & continued on through the night, but there were quite a few that changed after dinner, I guess to be more comfortable when walking the deck, shows, etc.
My sister just got back off the Carnival & I believe that it is not as formal as HAL ships. She did dress, but she said there were some that had jeans on in the dining room.
To be honest, everyone did their own thing. What works for you will be fine. Have a great time, we did. I'm curious as to the Disney line & how it is there. Happy cruising!
HAL Zuiderdam -October 2003
Disney Wonder - May 2004
HAL Statendam - May 2005
Disney Wonder - Sept 2006
Carnival Fantasy - Dec 2006
Carribean Princess - Oct 2007