According to estimates based on carrier's numbers, only 2/3 of iPhones are "legally" activated, which means 1/3 of all iPhones sold are SIM unlocked and being used sans carrier contract. Here's how they arrived at this number.
First, AT&T put out a press release saying that 2 million iPhones were subscribed on their service by the end of 2007. The three European carriers with iPhones (O2, Orange and T-Mobile) don't have official numbers, but iLounge rates them at about 300-400k, which means the total is only around 2.4 million. This, with Apple's announcement that 3.7 million iPhones were sold in 2007, makes put the number at about 1/3 of all iPhones being unlocked.
Even if the European iPhone numbers are low and AT&T customers haven't activated their Christmas phones yet, that still puts unlocked iPhone numbers somewhere around 1/3 to 1/4, which is extremely high by any count. [iLounge - Thanks David!]