The MacBook Air seams to be running in short supply at Apple stores and Resellers. The reason for short supply is unknown, it could be because the Air is selling like hot cakes or due to short supply of components. ArsTechnica noted that when Apple does get shipments of the Air it sells out within that day.
"After a full month of being on the market, the MacBook Air is still a difficult commodity to obtain in some markets (not all), and Apple is still quoting a 5-7 business day minimum shipping time even for the most standard models ordered from its web site."
"Meanwhile, Amazon won't be shipping orders until March 16 now, and many customers who run to a Best Buy thinking they'll find one hiding out there will be surprised: there aren't many to go around. One Best Buy source (not a retail floor worker) told me that Apple has kept them pretty thin on stock."