According to mobile advertising company AdMob, the iPhone's world-wide smartphone market share has reached 40% in just over two years. Meanwhile, competitors Nokia and RIM lost ground.OK, before you cry foul, remember that AdMob serves ads. This means that 40% of the traffic it sees from its ad network is coming from iPhones. It doesn't mean that the iPhone represents 40% of all smartphones that are actually in the market.

That said, the trends paint the real picture here. The iPhone's presence around the world grew from 33% to 40%. That's an increase of 21%. In North America, the iPhone grasps some 52% of the market. The increase in market share is more important than how many devices are actually being used.

At the same time, Symbian OS's market share dropped around the world from 43% to 34%. That's a 20.9% drop in market share. Interesting how Apple's market share went up 21% and Symbian's dropped nearly the exact same amount.

Other market share losers were RIM (from 10% to 8%), Windows Mobile (from 7% to 4%) and Palm OS (from 3% to 1%). I am somewhat surprised to see RIM's drop in market share, as it continues to have top-ten-selling smartphones. Its no big surprise to see Windows Mobile falter a bit. Everyone knows that Windows Mobile 6.5 is coming out soon, and it's not unreasonable to suspect that people are waiting for it to be released. Last, Palm OS is a dead OS that is being replaced by webOS.

Aside from the iPhone, the only other mobile platforms to increase in market share were Android, which grew from 2% to 7%, and Palm's webOS, which jumped from 0% to 4%.

All of these changes occurred over the last six months, says AdMob.