There are an estimated 4 million 'Jailbroken' iPhones in circulation

"It’s been months since we introduced jailbroken detection to our library, and we’ve begun to get a better understanding of the jailbroken ecosystem, which has remained reasonably steady. To date, Pinch Analytics has received data from approximately 4.0 million jailbroken devices. About 38%, or around a million and a half of those, have used a pirated application. This estimate is low – application pirates take steps to avoid detection – but it’s worth pointing out that an individual who jailbreaks their phone is not necessarily an individual that steals applications."
Source: Pinch Analytics, 12th October 2009