First the Amazon Kindles sell out. Are iPods next?

Apple is struggling to keep up with the demand for iPods, which could lead to a shortage during the holiday season, according to a Wednesday research note from Kaufman Bros. analyst Shaw Wu. Amazon.com, for example, recently pushed the availability of the 8GB second-generation iPod touch from 11 days to 5 weeks. The 16GB model is still at 3 to 5 weeks.

"In addition, from our distribution and channel checks, iPod touch, iPod nano, and even iPod shuffle are seeing spot shortages on different colors and capacities across several retailers including Best Buy, Target, Wal-Mart, and Crutchfield.com," Wu wrote.

"Frankly, we find these sell-outs on iPods surprising given how difficult the macroeconomic environment is, putting a crimp on consumer spending," according to Wu. "From our assessment, we believe iPod is holding up better than most, due to its relatively low [selling price] and strong consumer understanding of the value it provides."

He predicted that Apple could sell 21 million iPods during this quarter.

News that Microsoft Windows market share has dropped below 90 percent while the Mac OS X increased to 9.25 percent "further validates our key investment thesis, which is that [Apple] is still an early platform adoption story with plenty of headroom for growth, driven by iPhone and Macs," Wu wrote.