The Windows Phone 7 Series UI Design and Interaction Guide is available for download as a PDF from from microsoft.com here.

From the introduction:

The Windows® Phone 7 Series CTP User Interface (UI) is based off a Windows Phone design system internally codenamed “Metro.” The Metro design principles center on a look that uses type to echo the visual language of airport and metro system signage. The goal is to clearly direct end users to the content they want. 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. A clear, straightforward design not only makes an application legible, it encourages usage. This guide will provide design knowledge and fundamentals for this type of UI development. We highly recommend that developers adopt the Metro design style whenever possible. Although requirements may vary based on the application, paralleling this experience will create a more consistent, fluid UI experience from the custom and built-in application view.

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. By understanding and incorporating key design concepts and considerations within these areas, you can craft your application to provide a better end user experience. You will also have a deeper understanding of the number of different hardware and software interaction elements that are available to developers in Windows Phone 7 Series CTP. Diverging from the Windows Phone 7 Series CTP interaction model is generally not allowed. However, there are a few exceptions where the UI behavior is different as application requirements vary.

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, and please give us feedback by sending mail to wp7des@microsoft.com. We will monitor that mailbox regularly, but please do not expect responses. If you want to discuss the design or guidelines please head to the Windows Phone developer forums here.