Absinthe 0.3, an iPhone 4S and iPad 2 (iOS 5) untethered jailbreak tool, has been released a few days ago and it includes a Linux version too.
iOS jailbreaking is a process that remove limitations imposed by Applet on its iOS devices, through custom kernels. Jailbreaking is required if you want to install software without the App Store.
I don't have an iPhone 4S or iPad 2 to test this on, but judging from the video posted by the folks from idownloadblog, it seems pretty easy:
As you can see from the video above, if you run Ubuntu, you basically only need to install the latest libimobildevice from its PPA (for old Ubuntu versions only), then launch Absinthe and start the jailbreaking process.
For step by step instructions on how to jailbreak iPhone 4S and iPad 2 on Linux, see the post @ idownloadblog.com
Download Absinthe 0.3 (available for Linux, Windows and Mac OSX)