Sure, the MacBook Air is a niche product, but it seems to be a darned popular niche product. The folks who are most interested in the MacBook Air are the kind that obsessively track the availability of their latest gadget crush, and now Apple has made that much easier for those jonesing for a little MacBook Air action.
Just load up the MacBook Air Availability page and find out if there are any in an Apple Store near you. As you can see above, my local Apple Store in Ardmore has some MacBook Airs in stock (sadly, the webpage doesn't list how many MacBooks are available, though it does point out that they are available on a first come, first served basis).
You might recall that Apple posted a similar page to track the availability of the iPhone.