This blog post links to the Windows Phone 7 Series UI Design and Interaction Guide, which is probably of interest to you.
If you are building a native app, you probably want to make it in accordance with the device's UI guidance. An exception might be if you are making an application that provides an interface to a website (like the Twitter and Facebook applications for iPhone and Android) - here, you might want to design it to look like you website to provide familiarity to users.
The decision to follow or not follow UI guidance is yours entirely. Although it's typically a good idea to present users with a familiar interface of some kind - either another (web or desktop) application or the phone's interface.