Piper Jaffray’s (PJC) Gene Munster was the first Apple (AAPL) analyst to dissect the domestic retail sales data released Tuesday by the NPD Group. In a note to clients issued shortly after noon, Munster points to several results he sees as positive signs for the company:

• Mac sales up 37% year over year for the first two months of Apple’s third fiscal quarter, which by his reckoning suggests Q3 Mac sales of 3.1 to 3.2 million units, slightly ahead of the Street’s consensus of 3.1 million.
• iPod sales down 13% year over year for the same two months, implying sales of 9 to 10 million units for the full quarter, slightly below consensus.
• Given that the iPad was on sale for nearly the entire two month period, Munster believes these results show minimal cannibalization of Mac sales and slight cannibalization of iPod sales. Given that iPads sell for four times as much as iPods, he sees this as “a net positive for Apple’s business.”

His conclusion will be instructive for anyone who wonders why the iPad doesn’t have, say, a camera or a physical keyboard.

“Apple has successfully limited the iPad functionality to primarily content consumption, versus content creation on a Mac.”

In other words, the lack of a camera or keyboard on the iPad gives customers a reason to keep buying Macs.

Below: Munster’s NPD charts.

[Follow Philip Elmer-DeWitt on Twitter @philiped]