Zacks Investment Research upgraded Apple's stock to "Outperform" from "Neutral" on Monday, based on the company's "earnings momentum and accelerated revenue growth." The investment firm largely attributed both factors in Apple's performance in part to the growing Mac platform and in particular to the success of the iPhone.

"We expect the iPhone-driven momentum to continue, as the company's product generates the highest customer satisfaction in the industry," Zacks' analyst said in a statement. "In our opinion, this is the key to long-term growth and continued appreciation of the stock, given the large addressable market."

The analyst also cited Apple's product shift to higher-margin iPhones away from lower-margin iPods, which has helped the company's gross margins grow from 34.3% in 2008 to 36% in 2009.

"Year to date, Apple's share price has more than doubled," the firm said. "Apple's valuation premium is justified, given the company's various positive attributes and track record in earnings growth, and leaves room for further upside from the current levels."

Shares in Apple traded lower in the Monday afternoon session, trading at US$188.16, down $.34 (-0.18%), on moderate volume.

*In the interest of full disclosure, the author holds a small share in AAPL stock that was not an influence in the creation of this article.