International expansion of the iPhone is the single most important driver of Apple stock in 2010, with overseas sales predicted to account for 80 percent of all of the company's handset sales by the end of 2010, one analyst believes.

Brian Marshall with Gleacher & Company (formerly Broadpoint AmTech) issued a note to investors Wednesday, in which he increased his 2010 iPad sales estimates to 12 million in calendar year 2010, and 20 million in 2011. And while those figures are astounding for a new product cateogry, the analyst said that the iPhone is still in the "driver's seat" for Apple.

That's because Apple's international carrier partners offer a postpaid subscriber base more than six times larger than AT&T, the exclusive wireless carrier of the iPhone in the U.S. Despite that, international sales in calendar year 2009 represented only 60 percent of total units sold.

Out of 65.1 million potential postpaid subscribers in the U.S., Apple has a market penetration of 4.1 percent. Overseas, with its 150 carrier partners, Apple could access 460.7 million subscribers, but it only had a penetration of 1.3 percent as of March.

Marshall sees the international numbers growing even more by the end of 2010, making overseas sales responsible for 80 percent of the iPhones sold this year. He has "conservatively" estimated that the company will sell 40 million iPhones in calendar year 2010, and another 50 million in 2011.

To put the numbers in perspective, last quarter Apple sold 8.75 million iPhones in a three-month span, the best-ever quarter for iPhone sales. To achieve 40 million in sales by the end of 2010, Apple would need to far exceed those numbers.

Sales will no doubt be bolstered by this week's launch of the iPhone 4 in the U.S., France, Germany, the U.K and Japan. Last week, Apple revealed that more than 600,000 handsets were preordered on the first day of availability.

While the iPhone remains the most important aspect of Apple's business, the iPad represents a significant growing market, Marshall said. On Tuesday, the company revealed that it sold 3 million iPads in the product's first 80 days of availability.

"Amazingly, in its first quarter of introduction, we believe the iPad family will exceed $2bil in revenue (roughly 14% of our estimate of $15.1bil for AAPL's total revenue in the June '10 quarter)," Marshall wrote. "Recall it took the iPhone four quarters to cumulatively reach $2bil+ in revenue."