what do you THINK should be worn?
All of the comments about people wearing virtually anything even to a 'nice' meal are correct but, IMHO, that does not make it 'right', or mean that you should also be under dressed for a special meal.
At Victoria & Albert's I only 'must' wear a jacket, but I also wear a tie - obviously not because I must, but because I want to - for my wife - who always dresses beautifully for such an occasion, and because it is part of an elegant and special experience. Similarly, no one has to force me to wear nice slacks and shirts at Jiko or California Grill - I do it because its part of the tradition and experience of elegant dining - or at least I feel like it should be. There are more than enough table service and casual dining restaurants at the World for if you don't want to get dressed up or be in too 'formal' an environment - if you are spending your time and money to dine at one of the 'signature dining' restaurants, don't you want it to be something special and memorable? Do you want to be like that table next to westie "filled with people in wet swimsuits"?
I guess the 'exception' is dining in the parks during the summer months - I think its a different story when its really hot and humid and you are walking around the parks, and in that case, comfort dictates more lattitude ....
However, this is for the most part a personal choice issue - and I'm probably in the minority in thinking a 'fancy' meal should be accompanied by a decent outfit ... but my wife is sure glad this is how I feel about it