The IMEI (International Mobile Equipment Identity) number is on all GSM and UMTS mobile phones, including the iPhone. The enterprising geeks at AFB and Investor Village have been collecting every iPhone IMEI they could get their hands on and adding them to a Google speadsheet.
After performing the voodoo that such a spreadsheet allows, researchers have figured out that Apple has manufactured at least 5,649,000 iPhone 3Gs. Add that to the 2.4 million 1st generation iPhones sold in 2008, and you arrive at 8 million iPhones.
That's a lot of iPhones.
These are unofficial numbers, of course. But if these folks are correct, Apple will reach their goal of 10 million iPhones sold in 2008 with room to spare.