When it comes to smartphones, the iPhone is still the hottest. The embodiment of the trendy gadget, Apple’s cellular cash cow continues to increase its market share quarter after quarter. The 4th quarter of 2009 saw almost 7.5 million iPhones sold, and it’s predicted that the first quarter of 2010 will see an increase of over 3 million sold. But how did the iPhone become such a juggernaut, even considering the historic crazed for location-based technology? Building off the buzz from our new (and free) guides app that integrates with Foursquare, the graphic below takes a look at the iPhone’s evolution — from its humble origins as a PDA to its current status as an app-fueled communications powerhouse, the iPhone is clearly here to stay. And that’s how we like it.