By Aidan Malley

Published: 06:15 PM EST

With more than 25 percent of total iPhone sales potentially in the hands of customers who don't subscribe to AT&T, investors should look to pure unit shipments rather than carrier numbers as a gauge for iPhone success, according to a note from American Technology Research.

Analyst Shaw Wu cites estimates that suggest "at least" 1 million and as many as 1.5 million iPhones are currently in use with an unauthorized provider, exceeding claims by BusinessWeek sources that estimated 1 million hacked units at a maximum. The phone is most likely in unofficial service in between 35 and 40 countries versus the official four, Wu reckons.

The discrepancy may be wide enough to mask underperformance in official numbers, with AT&T's official subscriptions and overall shipments both relatively low. While predictions at the research firm call for an above-consensus 2.1 million iPhones to ship in the first calendar quarter of 2008, it may be the up-front price for the device and not its shared subscription revenue that carries the day.

"We are not sure if looking at AT&T additions is the right way to measure sales as we believe the hacked iPhone market is much larger than expected," Wu says.

Instead, Apple's current success is said to be measurable through the $399 in revenue collected from each phone as well as first-party accessory sales. The Cupertino, Calif.-based firm also is not as heavily dependent on the iPhone for revenue: having accounted for just 3 percent of Apple's latest quarterly revenue, even a complete shutdown of iPhone sales would result in just a 3-cent drop in Apple's earnings per share, the analyst says.

Mac sales may help soften any perceived blow. Though iPod shipments may be weak during the current quarter, with a steep seasonal decline dropping sales to an estimated 9.7 million units, the MacBook Air is believed to be boosting traffic both for Mac sales and for retail stores. Despite "underwhelming" numbers, the MacBook Air could help Apple reach 2.1 million Macs sold in the first three months of the year.

The larger picture leads American Technology to predict that Apple will track above its own financial guidance and has buffered itself against a recession.

"We continue to believe AAPL is well-positioned to weather the storm better than most with its strong fundamentals," Wu notes.