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NPD market survey, reported at AppleInsider notes that Mac OS X 10.5 Leopard is the fastest-selling Mac OS upgrade in history.

2 million copies of Leopard were sold within the first 48 hours of retail sale, but Leopard's first full month on sale beat all previous records, although NPD did not provide figures.

With OS X 10.4 Tiger, Apple expected to reach 2 million sales after 43 days. That was blown out of the water by Leopard. To date, 10.5 is selling 20% better than 10.4, which itself sold 30% more units than OS X 10.3 Panther. Going even further back, Panther doubled Jaguar (OS X 10.2) sales.

"It's really stunning to see Apple have one blow-out OS launch after another," NPD analyst Chris Swenson told AppleInsider. "It's clear that Apple has hit upon the right strategy for rolling out new versions of its OS."