I've been looking at the spell too. (I'm a player in mypetrock's campaign.) The effect doesn't appear to be very well defined, but my impulse is to contrast it against Gaseous Form.
GF states all kinds of limitations and side effects: the target loses the benefit of material armor, can't manipulate objects, loses supernatural abilities, becomes immune to critical hits, etc. He can still cast spells, but only if they are metamagicked to have no V/S/M/F components.
Wind Walk only states that the targets can't cast spells. It doesn't say that it prevents supernatural abilities or physical attacks. Probably it should have more limitations, because it appears to be intended only as a utility spell, for travelling long distances. As written, it also makes a decent combat buff for non-casters. I can't find anything about it in the errata or the FAQ, though.
Anyone else have some input?