BJ is right - you would probably feel uncomfortable. We only saw one person in jeans (and I was looking after so many posts about jeans), although there was a guy who wore do-rags on all nights except the formal night.
He actually had quite the collection and wore a different color every night!
Carnival Elation March 11
Carnival Imagination Sept 07
Carniival Sensation Dec 06
RCI Sovereign of the Seas Sept 06
Carnival Miracle Sept 05
Carnival Glory Sept 04
Carnival Fantasy Jan 04