The well known jailbreak developer Chpwn (aka Grant Paul) tweeted a screenshot of Cydia running on an iPhone 5 the same day the device was released. While this is exciting news, it does not mean an untethered iOS 6 jailbreak is right around the corner. It took the jailbreaking community a couple of months to publicly release Absinthe 2.0, even though three different jailbreaks were reported to be working on the new iPad before its release.
This hopefully means the jailbreaking community is also on the road to jailbreakng other iDevices running iOS 6.
Chpwn released a second picture of the device to prove the jailbreak is not a fake. You can clearly see from the photo that it's indeed an iPhone 5 running iOS 6.0 and Cydia 1.1.