The GreenPois0n Absinthe software uses a "ridiculously complex combination of exploits-within-exploits" to jailbreak the iPhone 4s and iPad 2 Apple devices running on the A5 chipset, according to a blog from the Chronic Dev Team, one of the iOS hacking groups.
Hackers from the Chronic Dev Team, including group leader 'p0sixninja', collaborated with researchers from the iPhone Dev Team such as 'MuscleNerd' and 'Saurik' as part of an iOS-hacking "dream team", according to the blog post. The groups worked with other iOS researchers, including 'planetbeing', due to difficulties in hacking A5 devices..
Chronic Dev Team initially had little success in jailbreaking A5 chipset devices, so launched a tool called CDevReporter to collate crash reports, according to the blog post. The reports were used by the researchers to find holes in the iOS operating system.
Jailbreaking an App is not illegal. It lets you build and run software from outside sources, and it also lets you copy files without agreeing to the iTunes end-user agreement.
For more on this story, read Hackers jailbreak iPad 2 and iPhone 4S on ZDNet UK.