If you open a Windows Phone 7 project that was created with the initial release of the developer tools you will see a prompt similar to:
Essentially one of the updates in the April CTP is related to the enforcement of security policies for applications that want to make use of device capabilities. I’m expecting that this will be integrated into the Marketplace experience so that users who download your application will have to acknowledge that the application will have access to capabilities on the device.
Anyhow, to get your application to run, you may need to add the appropriate capabilities, as instructed in the warning message. From the previous release of the dev tools, your WMAppManifest.xml file (located under the Properties node of your Windows Phone 7 project) probably looks similar to the following:
<Deployment >http://schemas.microsoft.com/windowsphone/2009/deployment" AppPlatformVersion="7.0">
<App >http://schemas.microsoft.com/windowsphone/2009/deployment" AppPlatformVersion="7.0">
<App >Jaime Rodriguez has a good post on Windows Phone 7 capabilities security model