I know there have been a lot of posts, but just to update...we just returned from the last 4-day cruise, and in Trition's (where disney recommends a jacket), we looked around to see what the men were wearing.
We had the main (early) seating. I would guess perhaps 10-15% of the men had on jackets; about 25-35% had on ties, and the rest (about half) work open collar shirts or polo shirts.
And, there was absolutely, positively no reaction from the staff or other diners to those who chose to wear the casual clothing. We enjoyed dressing up, other tables enjoyed wearing their comfortable clothes, and we all had a grand time. I may have been the only person who even noticed the range of dress.