The new iOS 5 was not jailbreakable already, but now there is a possibility for the same, only if it is a tethered jailbreak. SemiTether makes things easier by making the phone rebootable without the need of getting it connected to the PC or Mac always when your iOS device shuts down without some reason or you voluntarily do it. SemiTether is a tool available through Cydia which bypasses the limitation of rebooting but would keep the jailbreak to just the name, and nothing much functionally.
Tethered jailbreak always has the rule of keeping the phone connected to the computer, so the Jailbroken iPhone, iPad or iPod touch with the iOS 5 would restart only when connected the computer but the need now would be gone because of the SemiTether tool. The phone would function normally as it did earlier, and even the apps would work, except the mail and Safari whose functionality changes after the tethered jailbreak is performed on the iOS device.
To use SemiTether and remove the limitation of connectivity for rebooting, you need to go to Cydia and go to sources and add – http://thebigboss.org/semitether
Adding that would install the new Semitether tool and the functioning starts. You can check whether it is working, by rebooting the phone without having it connected to the PC or Mac.