A new browser window will open with a radar weather map that takes full advantage of your iPhone's Touch OS. You can scroll around the map by dragging with your finger, use the pinch gesture to zoom in or out and rotate your iPhone for landscape viewing. This is the best implementation of a radar weathermap I have seen so far. You can bookmark the map to your bookmark list or to a home screen, the weather map will update when you launch it.
You also have the option of displaying Satellite VIS (cloud cover) or Satellite IR4 (detects clouds day & night) maps as well.
While this is a webapp, it outshines the mapping capability of any of the native iPhone weather applications.. or at least the ones that I have used. After checking this out, you may find yourself bookmarking the icon to your home screen.