And it continues to sell well. While Leopard and Tiger sales declined more than 60 percent after their first week at market, sales of Snow Leopard have only declined 25 percent in the two weeks it’s been available.

“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,” NPD’s Stephen Baker said in an announcement.

“With pricing reduced by more than $100 for both the single-user and five-user pack versus Leopard pricing,” Baker added, “Apple has clearly demonstrated that aggressive pricing policies in this economic environment generate an outstanding consumer response.”

In a research note issued earlier this summer, Piper Jaffray analyst Gene Munster estimated that Apple (AAPL) would sell about five million copies of Snow Leopard in the quarter in which it was launched. And while that number might have seemed optimistic at the time, it’s looking increasingly more reasonable. Leopard, as you might recall, sold more than two million copies its first weekend at market. If Snow Leopard is selling twice as quickly, Apple should have no trouble hitting Munster’s target and perhaps even passing it.

Among the 25 issues repaired in Leopard 10.5.1: a major one that resulted in a “potential data loss issue when moving files across partitions in the Finder” as well as other problems with Time Machine, Firewall, Back to My Mac, Airport and Mail.

The release of OS X 10.5.1 follows hot on the heels of OS X 10.4.11, Apple’s 11th and likely final update to Leopard’s predecessor, Tiger, which patched 41 vulnerabilities in the OS.

An impressive number, and one that took Mac OS X 10.4 Tiger–once “Apple’s fastest selling OS release ever”–39 days to hit. Said Apple CEO Steve Jobs, “Early indications are that Leopard will be a huge hit with customers.”

Certainly appears that way. “These numbers show the Mac user base is growing,” Piper Jaffray analyst Gene Munster said in a research note. “It also shows that it is an unusually active user base, with 9% of the approximately 23 million users upgrading in the first four days.” Clearly, Mac users are quite a bit more excited about Leopard than Windows users are about Vista.

Priced at $129 for a single-user license, Leopard offers some 300 new features–“Even Leopard innovations have innovations,” says Apple–among them, a solution to the “Fix the F-ing Finder” problem, a significant update to Mail, a virtual desktop application called “Spaces” and Apple’s eagerly awaited backup solution, “Time Machine.”

Looks to be an impressive release. Certainly, Wall Street expects it to be. Just last week Piper Jaffray & Co.’s Gene Munster predicted Leopard would add $240 million to Apple’s next quarter, nearly double the $125 million in revenues that Tiger brought in during its opening quarter.

Chris Swenson of the NPD Group, too, figures Leopard will outperform its predecessor. “Tiger was the best launch of Mac OS X ever according to our data,” he told ComputerWorld, “and I expect Leopard to do even better.”