Last year Cafe De Artists had a dress code of a collared shirt for men (could be a polo shirt or short sleeved silk or dress shirt) and women needed to be in something like a sundress, skirt or dressier type outfit (resort wear).
I think for the most part, most restaurants will still let you in, although you may feel a bit out of place in the finer restaurants in a t-shirt and shorts. I just dined at De Santos last week and the majority of the clientele were locals and Mexican Nationals from Guadalajara and everyone was dressed very elegantly. (Part of this is because after dinner, most people go to the Terrazo Bar or the disco.)

If you are coming down in the winter (Dec..thru Feb) you may want to consider packing a pair of long pants as it can get chilly at night, as well as they tend to crank the A/C in some restaurants.

Have fun, the new Bar Constantini is nice, and they have a new set price meal menu...plus the new renovations and additions are very beautiful!