I have the PS confirmation sheet from the trip we took in February 2004. The confirmation sheet included a "Hints and Policies" sheet, one of which is the designation of "Dress Code - Dressy Casual." This is described as "Collared shirt prefered for gentlemen. No tank tops, t-shirts, cut offs or jeans."
However, of the restaurants we went to, the only restaurant this dress code was applicable to was Citricos and it did not apply to Flying Fish, Spoodles, Bistro de Paris, Jiko, Yachtsman Steakhouse or Artist Point.
Thus, as others have said, there is no official dress code even at many of the high end restaurants, although I personally always wear at least khakis and some sort of collared shirt at all such restuarants.