Brian; The short answer is yes. Different fabrics have different levels of spf ("sun protection factor" the same numbers you find on sunscreen), thin cotton like a t- shirt only has an spf of about 5 and this number goes way down when the fabric gets wet. the reason is that more uva and uvb rays can penetrate the wet fabric. Granted wearing a wet t-shirt is better than nothing but not by much. This is why a lot of tournement fishing pros have started wearing clothing specificaly designed to have a higher spf number. There is a really good article about this in the June 05 issue of Field and Stream. Now don't get me wrong I still go out in shorts and t-shirts and I still ocasionaly get a little sunburn but I try more now to use more protection when I know I am going to be spending alot of time in the sun. Some survival techniques are for short term survival and some are for long.
hope this helps answer your question