my guess is that most of the people that already went with an Core2Duo Air, can get by without too much CPU horsepower, i.e. they're not doing intense video/audio work, for example, so, for many, it wouldn't even make sense to upgrade. (and, there are some who would rather die (or at least kill) than give up their 320M.)
... there seem to be two camps on the MBA forum these days:
1. Current Owners: Don't need much CPU, (sometimes think nobody should), like their GPU (well, some have gone from 'like' to calling non-stop, hiding in the bushes, ...).
2. Sandy Bridge Anticipaters: Want the form factor of the Air (and price and resolution), but simply need more CPU.
many of the threads on this forum, including a lot of the fights, i think, can be understood by understanding the needs of these two populations.
there are very, very sensible and knowledgeable people on both sides. and there are some people who could improve a bit on these fronts.