Snow Leopard sales "far exceed" the sales of Leopard and Tiger in the first two weeks of availability compared to the first two weeks of availability when the earlier operating system upgrades were released, according to NPD Group. Not only was the first week substantially better for Snow Leopard sales, the firm said that sales fall-off for the second week was only 25%, compared to 60% for Leopard and Tiger.

"Even though some considered Snow Leopard to be less feature-focused than the releases of Leopard or Tiger, the ease of upgrading to Snow Leopard and the affordable pricing made it a win-win for Apple computer owners - thus helping to push sales to record numbers" Stephen Baker, vice president of industry analysis at NPD, said in a statement.

The firm pointed to the reduced pricing of Snow Leopard ($29, compared to $129 for Leopard and its predecessors) as a catalyst for increased sales, but another important point is that there are also many more Macs on the street eligible for upgrades than there were when either Tiger or Leopard were released. Apple has enjoyed record Mac unit sales for much of the past three years, increasing the size of the user base every quarter.

NPD also hazarded a look ahead at the release of Microsoft's Windows 7 OS later this year: "As we head into the fall selling season, and the release of another major OS upgrade [Windows 7], it will be instructive to see if that upgrade - currently projected to sell at ASPs much higher than Snow Leopard - can deliver the same incremental increase in consumer demand that Snow Leopard has enjoyed."

Whether or not Windows 7 can see the same incremental increase in demand as Snow Leopard, Big Redmond's successor to Vista will still vastly outsell Apple's OS simply because of the size of its market, which is 10-12 times as big as Apple's.