In reply to blackreaver:
The face fabric of down jackets has a very tight weave to prevent the down leaking out as much as possible, this has some impact on the breathability. Fabrics for synthetic insulation don't need to be quite as tightly woven, so are slightly better for breathability.
As Shearwater says, Polartec Alpha is a new synthetic insulation that can cope with much more breathable fabrics. See:
http://polartec.com/warmth/polartec-alpha/ How much better is it? No idea!