For the first time, there are more smartphones based on Google’s Android operating system accessing the AdMob mobile advertising network in the U.S. than there are from the iPhone, according to the company’s March data.

Here’s the data: Android phones have a 46% share of the U.S. smartphone market while the iPhone has 39%. But there are a number of caveats to point out.

First, AdMob gets its data not by number of handsets sold, but by tracking which operating systems access its mobile advertising network. And since most of Android apps are free, its developers rely more on mobile advertising for revenues than those making paid apps for the iPhone. Also, it should be noted that AdMob is owned by Google, which acquired the company last year, although the deal is not yet complete.

Regardless, the findings are not that surprising. Apple is the only one making iPhones while several phone manufacturers are creating Android-based devices. There’s simply more versions of Android available than iPhones, so it’s inevitable that Android would overtake the iPhone in share.

But these figures should not be confused with overall market share in terms of handset penetration. After all, it doesn’t even list Research In Motion’s BlackBerry. The most recent comScore data from February listed BlackBerry as having 42% of the smartphone market, compared to Apple’s 25% and Android’s 9%.

Number of devices aside, Apple still leads the way in app downloads and usage. The Wall Street Journal today reports that about 1 million apps a day are downloaded from the BlackBerry App World by about 20 million registered users. Meanwhile Silicon Alley Insider estimates Apple sees between 10-20 million apps downloaded a day from the iTunes App Store.