The iPhone 5 is speculated to be running Apple's new A5 chip that is already present in the iPad 2. At the jailbreaking convention "My Great Fest", Chronic Dev Team member p0sixninja had announced that the A5 chip has 5 userland jailbreak exploits that can be used to achieve an untethered jailbreak on all A5 devices, which would make the iPhone 5 jailbreakable as well as the iPad 2.

“it’s the record breaking number of exploits found” said p0sixninja at the convention. 5 jailbreak exploits should make the iPad 2 and iPhone 5 jailbreak-able very easily and it means that we can expect a jailbreak for the iPhone 5 very soon after it is launched. However the Chronic Dev Team said it still needs to check the exploits on the iPhone 5 itself (after launch) to make sure its jailbreak-able but hopes are high that it will be. Plus an entethered iOS5 jailbreak should be excellent since you would not have to connect it to a PC everytime you restart the iPhone/iPad.