The Windows Phone 7 UI Design and Interaction Guide is available from Microsoft for download. Update: This guide had been updated to v 2.0. Please see link at the end of the post.
The Windows® Phone 7 Series CTP User Interface (UI) is based off a Windows Phone design system internally codenamed “Metro.”
Metro interfaces are supposed to embody harmonious, functional, and attractive visual elements. Ideally, good UI design should encourage playful exploration when interacting with the application and people should feel a sense of wonder and excitement.
This guide will provide design knowledge and fundamentals for this type of UI development.
This guide will also detail several possible methods of interaction that can be used by a Windows Phone 7 Series CTP application, including standard input, functionality within the UI framework, and the Metro-themed Silverlight® and system-based controls.
The guide has 69 pages of prescriptive guidance on how to design great Windows Phone applications. It’s a preview of the final documentation, so expect tweaks over time.
UPDATE: Download Windows Phone 7 UI Design and Interaction Guide v 2.0.