Ben Willi explains how to create custom page transitions in your Windows Phone 7 applications.
Source: The Brain Dump
The Silverlight for Windows Phone Toolkit provides the mechanism for creating animated page transitions in your Windows Phone 7 applications. It also provides a few standard transitions that match the look and feel of the built in apps such as pages sliding in, rotating out, etc. But what if I want to create a new transition that doesn’t exist in the toolkit? What if we want to run an arbitrary storyboard as the page transition? Then we need to build a new class that fits into the toolkit’s page transition framework.