In this post, Laurent Bugnion explains how to get a screenshot of a WP7 application's page.
Source: Laurent Bugnion's Blog
Often times, you want to take a screenshot of an application’s page. There can be multiple reasons. For instance, you can use this to provide an easy feedback method to beta testers. I find this super invaluable when working on integration of design in an app, and the user can take quick screenshots, attach them to an email and send them to me directly from the Windows Phone device. However, the same mechanism can also be used to provide screenshots are a feature of the app, for example if the user wants to save the current status of his application, etc.