This was in March 2010 but since then the Windows Phone 7 Developer Tools Beta has been released and with that the icons are now included as a drop down option in Expression Blend which makes the creating your UI a little easier.
At MIX2010, during the Windows Phone UI and Design Language session the following slide was shown which had a few extra icons.
I needed the ‘car’ icon but wanted it in vector form so I could resize nicely. So I converted these images to Illustrator format to be more easily used.
In any case, you can download the icons here.