The iPhone 4S has boosted Apple's (AAPL) fortunes. The stock has recently hit all-time highs and was hovering Wednesday morning at nearly $542, up from around $400 in October last year when the iPhone 4S was launched.

Not only has the iPhone 4S helped Apple become the No. 1 global smartphone maker, but it has also helped narrow the gap with Google's (GOOG) Android operating system in the U.S. in the last quarter.

Apple led the mobile ad impressions market in North America with a share of 35% from October 2011 to January 2012. Clearly, the launch of iPhone 4S helped Apple rise up to Android. Research In Motion's (RIMM) BlackBerry operating system saw an even steeper decline than Android, dropping 8.9% to finish at third place with 11.6% share during the same period.

Mobile usage directly depends on smartphone popularity

Apple's growth in the mobile ad impressions market is a direct result of the iPhone's increasing popularity. According to one report, the iPhone 4S, iPhone 4 and iPhone 3GS took the top three spots in the U.S. smartphone market last quarter. And iOS, with a market share of 43%, narrowed the gap, with Android at 48% in the smartphone operating system market.

Even globally, Apple is now the leading smartphone arena. It recently overtook Samsung to gain the number one spot with a market share of 24% in Q4 2011. This goes to show that the more popular the smartphone, the higher is the mobile web and apps usage. The increase in this usage results in increased mobile ad impressions.

Our $550 price estimate for Apple's stock is about 5% above the current market price.