The process of unlocking your WP7 device for development is very easy.
- connect your device via USB cable
- launch the tool “Developer Phone Registration” from All programs > Windows Phone SDK 7.x (see the Figure 1)
- enter your Windows Live ID and password (if your phone screen is locked, you’ll have to unlock the screen and retry it.)
Note: unlocking your WP7 device using the tool allows you to debug and test applications on your device.
For more details see http://msdn.microsoft.com/en-us/library/ff769508%28v=vs.92%29.aspx