Rumor has it that a new homebrew app will soon open the full Bluetooth stack on the iPhone 3G, allowing for full and proper Bluetooth support including wireless keyboard connectivity and stereo Bluetooth. The app is called iBluetooth and it’s currently available on the Cydia store.
This current app supports Bluetooth file transfers and updated versions will allow you to use the rest of the stack. The programmers are asking for $3.99 so they can continue development on a more fully-featured system. Might be a worthy cause.