Not just once, but several times by different developers. One coder worked out to install iOS 5 without the need for a $99-a-year developer account, whilst another - Mert Erdir - posted images on his Twitter account showing the unofficial Cydia appstore running under the new iPhone operating system.
According to the Gizmondo newswire, Erdir managed to leverage a coding gap in the operating system's VoiceOver facility.
This leveraging approach - effectively exploiting coding gaps in the iOS 5. API overlay system - was used to great effect by George Hotz when he staged the first jailbreak of iOS shortly after the iPhone Mk 1 was unveiled several years ago.
The LA Times, meanwhile, reported that Apple's problems started when it released details of iOS 5.0 - apparently including a dev image of the install software/ROM - through a number of developer channels.
This is, says the paper, "customary with Apple in order to get an early look at new features and [allow developers to] begin updating their software accordingly."