With so much attention focused on the iPad, J.P. Morgan’s Mark Moskowitz chose Monday to change the subject.

He issued a note to clients that focused instead on the fastest growing member of Apple’s Mac line: The MacBook Air.

According to Moskowitz, revenue from Air sales tripled and units quadrupled year over year after Apple (AAPL) cut the price, added a camera and introduced an 11-inch model last fall.

He writes that demand for the Air remains “sturdy,” and that by fine tuning what had been a flawed product, Apple may have created a whole new category, one that falls somewhere between tablets and full-sized laptops.

“We believe,” he says, “the key attributes that distinguish the MacBook Air from standard notebooks and even tablets could help define a new category — ultra-portable devices for productivity users — within the PC market.”

Moskowitz estimates that the Air accounted for 4.9% of Apple’s Mac units and 5.4% of Mac revenue in 2010. If sales and prices remain steady, he says, the Air could easily generate $2.2 billion in 2011.

Below: His spreadsheets.