I don't care if wool is in or not. All i care about is what works best for me. And so far that is wool. I just love my Icebreaker.
The thing with synthetics is the cold flashes. When you stop to eat your powerbar or GORP or sth, the synthetics dry like crazy at the expense of your body heat. So every time i stop in a synth shirt i get cold, almost immediatly. That doesn't happen in my wool shirt. It keeps me warm at all times even when it's wet.
Despite what some on this site may say about that there are no fabrics that are warm when wet. Wool is. It can absorb about a third it's weight in water and still feel warm. A synth shirt cannot do that.
There is another, though not so important, good quality of a natural fabric. If in a rare case you are cought in a fire (leaking petrl stove causing a big flame) your synth shirt, made out of oil, will melt into your skin :S.
Only thing wrong with a wool shirt is that at times i tend to overheat. But that usually isn't a problem since i prefer hiking in cold whether.
I love my IB