I have an app on the iphone called plane finder that does a very similar thing.
My father is now a retired aircraft engineer, RAF for 22 years then civilian for the rest and he rarely goes outside without his binoculars for watching the planes going in and out of Gatwick.
In the summer he sits there watching them and saying what they aircraft they are and what airline. I sit there telling him the registration number, route, altitude, call sign Etc..... He wants an iphone now just for that!
Craig.