The ChevronWP7 team have been informed by Microsoft that the upcoming update for Windows Phone 7, likely to hit devices in February, will stop that route to developer-unlocking devices from working.
While there are likely other, more complex or hazardous methods, such as actual security vulnerabilities in the OS, browser and other components, this will stop the easy unlocking which has led to the nascent home brew community around Windows Phone 7.
The ChevronWP7 team are actually planning to meet with Microsoft to discuss this, and to maintain developer application security while allowing the side-loading of non-marketplace applications.
While this may seem paradoxical, Microsoft does in fact run a similar system on the Xbox 360, in the form of the XNA Creators Club, and providing an easy route for amateur hackers to get their applications on Windows Phone 7 may remove much of the incentive for those self-same hackers to crack the platform.
Read more at ChevronWP7 here.