I grabbed GoSkyWatch Planetarium when it was free for a day. It uses any iPad's gyro and compass, along with Location Services, to display what you're looking at, no 3G or wifi required. It's the only astronomy app I've kept, out of half a dozen tried. One reason is its minimum-magnitude slider, which allows matching the app to the sky you actually see, very helpful if you're not in the desert or on top a mountain! It also has a red 'night vision' mode, and larger type for labels than most of the others. It's just easier to use IMHO.
I may find a better one some day, but for now this is my favorite.
milliHelen: amount of beauty required to launch one ship.