I have created a stencil for wire framing Windows Phone 7 application in Cacoo.  This is definitely a work in progress, but until it is complete I would suggest combining this stencil with the Android stencil that is available by default in Cacoo.  Below are a couple of screen shots of the stencil so far.

First here is what the stencil window looks like currently.

Taking a closer look the main device frame is illustrated below

Lastly is the button pallet which contains the icons from the Windows Phone toolkit.

Check back and see more as other general controls are added to speed mocking your applications.  You can find the stencil here.