Windows Phone 7 Marketplace lets you peek at any app's source code

"It has just been discovered that it's possible to download any Windows Phone 7 XAP full application package directly from Microsoft's Marketplace server -- no need to use a Windows Phone 7 device or the Zune application. This is possible because the Zune application uses Atom XML feeds to get the applications' information, so it's fairly easy to grab a direct link to the full app package just by looking at the XML. Once you have the XAP, you can unzip it and get access to all the assets and resources that app uses. And, by using a tool such as Reflector, you may even get to peek at the app's source code. One thing of note is that the XAP can't be installed on a retail WP7 handset, although it can be used in an emulator."

Source: http://www.downloadsquad.com/2010/11/12/windows-phone-7-marketplace-lets-you-peek-at-any-apps-source-co/