Hi James:
The label on the 2006 Marmot Ion is completely wrong. At the risk of stating the obvious, there is no down material at all.
Marmot's website states that the 2006 Ion is made of Pertex Quantum. I am no expert at all, but some have voiced suspicion that it really isn't. Whatever it is, it feels kind of "plasticky" — like the old Golite Helios / Ventus jackets. It's also not very breathable.
The new (2007) Marmot Ion actually feels nice next to skin — silky and soft (although not as silky / soft as the much more expensive Patagonia Houdini). The MSRP has also come down by a whopping 50% — 2007 costs $50 while the 2006 costs $100 at full retail.
If money is no problem and you insist on the best (meaning lightest and most breathable windshirt) — then I would pick the $120 Houdini. On the other hand, the new Marmot is pretty good in its own right — and lots cheaper.