MacBook Air Selling Out
As noted by ArsTechnica, Apple's MacBook Air appears to be running in short supply both at Apple Stores and major retailers.
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.
The cause of the short supply is currently unknown: whether it be short supply of components or high demand for the diminutive notebook, however Ars notes that stores that do receive stock often sell out within a day.
See Also: Apple Store MacBook Air Availability Tool