My idea is a combination of Knocking Live Video and iPTT. Essentially, an app where you can send someone a message to chat through push notifications (Knocking Live Video) with you in a push-to-talk app (iPTT). Also thought it would be cool if you were able to talk to multiple people at the same time. Obviously, it would be able to work over 3G and through wi-fi.
This idea came about because some friends and I are planning on playing paintball over Easter. Hours upon days of playing Modern Warfare 2 made me realise that this could be a viable option since most of my team have iPhones.