Can I use 2 iPhones as walkie talkie? (without internet!) I want to use 2 (or more) iPhones as walkie talkie (i.e. they talk each other without needing internet/voice call).

Of course this should work with wifi, because it doesn't make much sense a to have bluetooth or usb connection for this application...
There's an app for that™.

WiChat will allow you to chat with the Bonjour protocol, that is, chatting with people on the local network without connecting to an external server.