I'd like to second the notion of wearing cotton May - October in the south west deserts. Synthetics wick precious moisture away at the gaseaous level keeping you dryer, and then they evaporate that moisture away quickly so that sweaty clothes dry in minutes instead of hours. In the desert, you need every precious drop of moisture your body can hang on to, and that includes using your sweat as a cooling system. When a cotton shirt absorbs moisture, the wet layer on your skin helps keep your body cool.
Not matter how hot it gets, in the summer I wear a long sleeve (sun protection) tissue-weight woven (no knits!) cotton tunic -- a short version of what you see people wearing in hot, dry African climates. I avoid knits because they get too heavy and clingy when wet -- woven cloth billows like a sail in the smallest of breezes, sort of like instant air conditioning.
Synthetic socks, with liners, on the other hand, are a must. Wet socks = blisters.
-Kim ("avatar" pic taken late November in Canyonlands, and am wearing a synthetic T.)