According to several unnamed sources cited by Fudzilla, the shortage of AMD's Radeon HD 5000 ("Evergreen") graphics cards is largely because the company didn't order enough 40nm wafers from Taiwan Semiconductor Manufacturing Company (TSMC). This is contrary to the original belief that AMD was simply shipping an overwhelming number of cards. The anonymous sources say that Nvidia ordered a significant portion of the wafers for its 40nm mobile chips.
Regardless of what is causing the shortage, AMD says availability will improve in the weeks and months to come, and in truth, they are probably in a position to take their time. While I don't doubt that at least a few sales are being lost by the shortage, Nvidia has yet to launch its competing DirectX 11 line -- plus there are few titles on the immediate horizon that employ the technology. Fudzilla speculates that despite AMD's shortage of Evergreen cards, this may be the company's best quarter in years.