It would seem the odds are good. The reported jailbreak was at the hands of chpwn, a well-respected developer, hacker, and jailbreaker.
On Friday, Grant Paul AKA chpwn posted an image on Twitter, which shows the iPhone 5 running the Cydia Store for jailbroken devices. He also posted the message:
"Taller screens like Cydia too. :) pic.twitter.com/fE14vXN2"
Naturally Photoshopping could have come into play, or even faked the Cydia icon by renaming a Web app and substituting the appropriate icon. However, when Stefan Esser, another iOS hacker, asked about it directly, Paul tweeted a response indicating that he had definitely jailbroken Apple’s latest and greatest super smartphone.
What's unclear is whether or not this is a tethered jailbreak or not. A tethered jailbreak is better than nothing, but it means each time the phone boots up it has to be jailbroken again. Last year, it took a few months for an untethered iPhone 4S jailbreak to arrive.
In addition to more tweeted images, chpwn posted the above image, which shows the phone, rather than a screenshot. That image had been requested as proof.
To solidify things further, MuscleNerd of the famed iPhone Dev-Team has retweeted chpwn’s tweet about the jailbreak. That pretty much confirms it as an official jailbreak.