“Why not just produce the icon in Illustrator or Fireworks and resize it the quick and painless way? Because vector graphics work best with sufficiently large resolutions. …Vector doesn’t scale down to lower resolutions as gracefully as one might wish.”

http://www.mezzoblue.com/archives/2007/02/21/icon_design/

http://mrgan.tumblr.com/post/708404794/ios-app-icon-sizes

http://www.firewheeldesign.com/sparkplug/2006/April/icon_design_bitmap_vs_vector.php