Apple is on track to sell more than 40 million iPhones and iPod touches this year worldwide. There are already 78 million iPhone and iPod touches out in the world today, but where are all these devices being used and purchased?

Google’s AdMob did some research into that question and found out that the US still has a strong lead in iPhone usage, with 50% of the world’s iPhones and iTouches being used in the US. The next four biggest markets were U.K., France, Canada and Germany. In total, 23 countries had more than 100,000 unique Apple visitors.

The US iPhone usage dominance may not last. Of all the countries where iPhones and iPod touches are used, the US had the slowest growth rate. The fasted growing countries between January 2009 and November 2009 were Japan, France, Australia and China. AdMob reports that Japan’s iPhone and iPod touch user growth has grown at more than 330% in less than a year.

What does this data mean for mobile marketers? Although the iPhone is most dominant in US markets, it may be worth investing in marketing campaigns for the iPhone in other markets, especially those that are fast growing.