Just 2 percent of all iPhone OS handsets still in use are Apple's first-generation iPhone, but a new calculation estimates that amounts to nearly 30 percent of the 6.1 million iPhones sold between June 2007 and July 2008.

Mobile advertising firm AdMob released its monthly Mobile Metrics report on Tuesday, which included information on the hardware and software used by owners of Apple's handsets in the month of March. The data revealed that just 2 percent of all iPhone OS-based handsets that accessed the AdMob network were the first-generation iPhone.

Philip Elmer-DeWitt of Fortune Brainstorm Tech took the data one step further, and factored in Apple's announced total of 85 million worldwide iPhone and iPod touch sales. After it debuted in June 2007 until the iPhone 3G came out in July 2008, the first-generation iPhone sold 6.1 million units, which is about 7 percent of the 85 million total handsets sold.

"If 7% of those iPhone 1Gs are driving 2% of AdMob's traffic," Elmer-DeWitt wrote, "that suggests that nearly one in three is still ticking — and visiting the Web."

Though the first iPhone has been supported since its release, Apple earlier this month announced that its forthcoming iPhone OS 4 upgrade will not be available for the first-generation device. In addition, the iPhone 3G, first released in 2008, will not have the mobile operating system's multitasking support available for the faster iPhone 3GS.

AdMob's latest report also revealed that the iPhone 3GS is the most popular iPhone, generating 39 percent of all its network's traffic in March 2010. That's up from the 30 percent total seen by its network in September 2009.

All versions of the iPhone represent 60 percent of the iPhone OS devices found on the AdMob network. The iPod touch represents the other 40 percent, with the second-generation iPod touch the second most popular device, behind the iPhone 3GS. The second-gen iPod touch accounted for 25 percent of iPhone OS devices in March 2010, beating out the iPhone 3G which was represented by 20 percent of Apple's mobile devices.