This icon pack from Templarian would easily take care of all your icon needs for Windows Phone development and elsewhere! The pack contains 48×48 icons and the source files so with Expression Design they can be resized and exported for other uses.
The 1.12 MB pack includes 300 icons released with Creative Commons license. If you find the icons useful, you might want to make a donation on the site. Interestingly, the man behind Templarian, Austin Andrews, insists that he’s available to create new icons as per the demand. Mention
@Templarian on Twitter explaining the icon. Wait a few hours for a reply including a screenshot of the new icon. At this time it should have already been included in the zip. Neat!
You might also find this visual tutorial by Austin on creating an icon for Windows Phone with no previous knowledge of Expression Design useful.