According to Digitimes, the 3000+ Athlon 64 is AMD's sweet spot while the dual core model is aimed at the lower cost end of the marketplace.
The Digitimes article at this URL claims that better than expected demand has caused the shortages.
But European motherboard firms, talking to the INQ on conditions of anonymity, were rather more blunt about the problem. One described the shortages as due to "bad planning". µ