In a note to investors, Piper Jaffray & Co. analyst Gene Munster said that the pent-up demand for Vista will put more Windows-equipped PCs in customers' hands in the January-March quarter of 2007. That, in turn, means Apple's share of the market will slip slightly, from 2.5 percent in December 2006 to 2.3 percent in March 2007.

Even so, Munster found that of 50 Best Buy stores in the U.S., 80 percent had sold fewer copies of Vista than they had anticipated. Still, 72 percent of those stores claimed that Vista is driving PC sales.

The Mac market share dip doesn't necessarily mean Apple will sell fewer computers. Instead, Munster said that the Vista-boosted uptick in overall PC sales gives Apple the opportunity to sell even more Macs.

"As PC buyers consider upgrading to Vista, which in many cases involves the purchase of a new computer, Apple is recognizing its opportunity to gain mind share with consumers," Munster said in his report. "The company views this season of Vista-related computer purchases as an opportunity to sell more Macs."

In its most recently reported sales figures, Apple said it had sold 1.6 million machines in the last three months of 2006, virtually the same number it sold in the previous three months.