It seems that jailbreaking Windows Phone 7 handsets isn't as straightforward as once thought, as the handsets feature a jailbreak-rollback feature that reverts the OS back to a locked state.
Developers of the now revoked ChevronWP7 unlock tool confirm the existence of the relock feature:
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.
After relocking, which happens after about two weeks, any sideloaded applications (that is, apps not downloaded and installed via the Marketplace) display the following message:
“[application name] has been revoked by Microsoft. Please uninstall it.”
Remember kids, it's not really YOUR handset.