The Windows Phone 7 Series development blog has published a set of icons for the app bar. These icons fits the UI theme of wPhone 7 and very useful helping you maintaining the native look and feel for your application. It´s a set of png icons for general functions, media content and hardware integration icons like camera etc.
The icons can be downloaded here. On the dev blog you learn how to add them to your application here.
Application Bar Icons for Windows Phone 7 Series for developers:
http://www.aha-soft.com/stock-icons/windows-phone-7-icons.htm
These icons are distributed as 48×48 png files with transparent background.