Microsoft has released design guidelines for Windows Phone 7, available here: http://msdn.microsoft.com/en-us/library/ff402535(VS.92).aspx. These are in document form, so if you’d like to write your own Silverlight and XAML application to follow the guidelines, you would have to go through quite a few steps to implement them yourself.
The Expression Blend team took the Windows Phone 7 Guidelines and turned them into some XAML files that you can use right off the bat. Combining different themes (including the iconing dark and light Windows Phone 7 themes), margins and paddings.
Blend also lets you see what your application pages looks like in different orientations (i.e. if you rotate your phone from portrait to landscape.
In this video, the Expression Blend team walks us through what Expression Blend can do for Windows Phone 7 developers using the Windows Phone 7 design templates, available on CodePlex here http://wp7designtemplates.codeplex.com. You’ll also need the Silverlight Toolkit for Windows Phone 7 (here: http://silverlight.codeplex.com/releases/view/52297)
The wp7designtemplates project is available as an Expression Blend solution you can open up and borrow from.
If you’ve got Expression Blend (check your MSDN subcription :) you can try this out yourself. Don’t forget to add references to C:\Program Files (x86)\Microsoft SDKs\Windows Phone\v7.0\Libraries\Silverlight\Microsoft.Phone.Controls.dll and C:\Program Files (x86)\Microsoft SDKs\Windows Phone\v7.0\Toolkit\Sep10\Bin\Microsoft.Phone.Controls.Toolkit.dll.
Here’s a simple app that I created :o)
(OK – I just had to copy the Grid I wanted to emulate from the CodePlex solution and paste it into one of my own XAML pages).