I did wear jeans twice on casual nights, with a nice blouse and shoes and saw many other people dressed the same (nice jeans, not holey or bleached out) - there was a young twenty something guy at our table who was asked to change because he was wearing Docker shorts.
The unfortunate thing is - some people can pull off wearing jeans and some can't - my mom and I can wear the same outfit and I am ready for a night on the town while she looks like she's been gardening...
Still - I think one night of dress up never hurt anyone and who doesn't look good and feel good when they're dressed up???
October 1997 Princess (Bahamas)
October 2000 Carnival Tropicale
October 2006 Carnival Miracle
October 2009 Carnival Legend