The iPhone Dev team has managed to create unlocking/jailbreaking software for iPhone 3G within a few days of that product's release.
The Pwnage software can also unlock the iPhone for use with other networks, the team said, "Just to clear up some confusion over what this actually does: yes, it jailbreaks and unlocks older iPhones, and jailbreaks iPhone 3Gs and iPod touches. We only support the 2.0 firmwares," the team explained.
The scale of international demand for the iPhone in countries in which the device isn't available is illustrated by the fact that by February 2008, estimates of the number of unlocked iPhones in circulation around the world ranged from 800,000 to 1.5 million.
Installing unauthorized firmware on an iPhone voids the warranty and can render the device useless.