In this video, Jeff Brand explains how to use message bus to make navigation between pages in your WP7 app when using the MVVM pattern.
Source: Channel 9
Do you have an MVVM-based Windows Phone 7 application and you don't like having to hook into the RootFrame to navigate between pages? Do you wish you could just "ask" to navigate to a new view from your view model? One possible solution can be found in this screencast.