The horn honking feature is part of the newly-released Zipcar App, which makes finding, reserving and controlling your Zipcar possible through the iPhone (and, over WiFi, the iPod touch). The controls are not as cool as Dr. Horrible's little app in the sing-along blog, but you can do more than honk. You can lock and unlock the car as well. The app wont' be able to unlock any car in the system – you still need to scan your Zipcard to start the reservation; only then is the app able to honk that horn. The App is free and available now in iTunes. You can take a tour here.
What we haven't seen in this app, but what would be a good idea, is a Google Maps feature that simply shows you where your automobile is. Also not as notable as the honking thing, but much more practical.