The font used in Windows Phone 7 is called Segoe WP, or Segoe Windows Phone. If you install Visual Studio 2010, this font comes with the download. If you don't have the VS and want the proper font to create graphics/images that mesh well with the Windows Phone 7 look, you can grab the font from XDA. Just run the executable (you may have to do it as an administrator), and that should do it!