Star Walk is an amazing iPhone app that gives you an interactive guide to the sky. The app takes advantage of just about every aspect of the iPhone (touch, accelerometer, and even the magnetometer and digital compass in the 3GS) to make constellation searching and identification as easy as looking up.
When you first launch Star Walk, it will ask if it can use your current location to determine what you can theoretically see in the night sky. Additionally, Star Walk will show you things not visible at night due to light pollution, especially when living in DC. But what you can see, Star Walk can identify for you.
Even when you can't see the stars (like during the day), it shows you what is up there and even what is below the horizon. It is mesmerizing. I can hardly wait when we go back to Maine, because the star view at night is amazing. Light pollution is not a problem then.
