http://shinyfastandloud.com/?p=52 
steadfastly Cotton, organic cotton and wool are all “natural fibers” but does that make them green? See the attached table with estimated energy and water usage for some common fibers. I’m hoping to find the same data on wool, bamboo, and PLA but even the cotton data is pretty surprising. Plus one of the largest footprints a garment will have in its life is the energy impact of washing and drying. Polyester and Polypropylene do not stain and dry almost immediately, so they require much less laundering and do not need to be dried. And polyester garments last for many, many years of hard use where others tend to wear out faster (in part due to heavy laundering).
There’s a place for every type of fiber (and sometimes nothing feels better than cotton), but just because polyester is oil based does not make it less green when you look at the whole picture (remember the ethanol hype). For full disclosure, I work with Polartec® which mainly uses polyester fibers, including an increasing amount of recycled-content polyester that saves significant energy compared to virgin polyester.