There are a variety of weather apps available for the iPad, but by far and away the best that I have seen is Weatherbug Elite for iPad. It has all the expected weather features, but what really blew us away is the mapping. First, we are using a Wi-Fi iPad and yet Weatherbug Elite is able to pinpoint our street location almost exactly apparently because they use technology from Skyhook Wireless that uses a vast database of Wi-Fi access points to triangulate your position.
Second, the radar and other mapping is what I would term infinitely scalable. You can use the usual iPad multi-touch to zoom the display in to your street and out to your state or larger. The other night we were in the middle of a line of thunderstorms complete with hail and funnel clouds. With Weatherbug Elite on the iPad zoomed in on our location it was like watching the storms pass overhead. Very, very neat.
However, the best news is that Weatherbug Elite for the iPad is free. Give it a try and see if you do not think it is slick as well.