The yet-unknown number of people who’ve bought a Windows Phone 7 device this fall probably didn’t know of a hidden feature on their devices: jailbreak rollback.

The jailbreak rollback reverts a jailbroken WP7 back to its locked state. It was observed by the makers of ChevronWP7, the first known jailbreak application for WP7, during discussions with Microsoft earlier this month.

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As creators Rafael Rivera, Chris Walsh, and Long Zheng wrote in a blog post:

“Contrary to circulating reports, Windows Phone 7 devices unlocked via ChevronWP7 are not being targeted by Microsoft. Instead, the phone is reverting back as a result of a periodic check. Simply put, the phone rings Microsoft and asks ‘Hey, am I supposed to be unlocked?’ If Microsoft responds with a ‘No, what are you thinking?’, the phone apologizes and initiates a lock down.”

“Unfortunately, while in this state, an unsigned application launch results in a misleading (and scary sounding) error message: “[application name] has been revoked by Microsoft. Please uninstall it.”

Launched in November, ChevronWP7 lets users side-load applications onto their phones without having to go through the Windows Phone 7 Marketplace.

On December 1, however, after “good faith” discussions with a Microsoft executive, the makers of ChevronWP7 agreed to discontinue the tool in order to show its support for Microsoft facilitating homebrew development.