Peter Kuhn has a short example demonstrating the use of the pinch gesture in WP7.
Source: Mister Goodcat
In my first article on Silverlight Show about XNA for Silverlight developers, I briefly mentioned the Gesture Listener of the Silverlight Toolkit that enables advanced gestures in Silverlight on the Windows Phone by using the XNA input libraries. Since then I've been asked about the pinch gesture in particular, so I'm providing a short example of how to use it.