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Munster estimates that the increase in Mac users since Tiger was released in April 2005 means that OS X 10.5 Leopard will generate almost twice as much revenue as the $125 million 10.4 Tiger accrued in the two months after its release.

"At that time, the OS X installed base was 12 million and Tiger sales added $125 million to the quarter," Munster wrote in a note to investors. "The Mac OS X installed base is now approximately 23 million, so we expect Leopard to add approximately

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$240 million to the December 2007 quarter. This assumes similar uptake rates to the Tiger launch, which saw 15% of the user base upgrade in just six weeks (eventually 66% of the user base upgraded to Tiger)."

He added that he also expects a slight upturn in Mac sales.

"While we do not believe many consumers delay the purchase of a Mac because of Leopard, we expect its new features and updated look to be positive for Mac sales," he wrote.

One reason why Leopard's may not have a particularly noticeable effect on Mac sales is that Munster, in common with fellow analysts at Citigroup and American Technology Research, believes that Apple shipped more than two million computers in the July to September quarter, easily breaking the record 1.7 million figure for the previous three months. Apple will to announce its most recent quarterly results later this month.