Can someone tell me if is available a series 60 application that allows you to use your mobile as a walkie talkie, using bluetooth or a modified version of the radio application that operates on the proper walkie talkie frequencies?
I haven't heard of such apps. Almost pointless with Bluetooth anyway, because of the max range being about 10 meters; you can talk to somebody 10 meters away without a walkie-talkie.
And no software application can modify the Bluetooth or cellular radio on the phone to operate as a regular walkie-talkie; it would require entirely different hardware.
The closest thing to a walkie-talkie like service like that is Push to talk over Cellular (PTT/PoC), which is a feature of some devices (and requires a corresponding operator service): http://www.nokia.com/nokia/0,,62518,00.html