This screencast covers making your App Store icon in Photoshop (I’m using CS3):
- Using a template to start your icon right
- How to use vector layers and position them precisely
- Creating icons at each of the required sizes and what to watch for
- Using Smart Layers for resolution independent files
- Using a preview window when working on small icons
Also note that the rectangle and rounded rectangle shape tools have this somewhat-hidden checkbox that helps to ensure your shape edges are right on the pixel-edge:
