how to route iPhone audio to the bluetooth headset

This little test worked for me... it involves setting up the bluetooth headset as the input also (not sure if that's what you want). Sorry about the crappy formatting on the code...

// create and set up the audio session
AVAudioSession* audioSession = [AVAudioSession sharedInstance];
[audioSession setDelegate:self];
[audioSession setCategory: AVAudioSessionCategoryPlayAndRecord error: nil];
[audioSession setActive: YES error: nil];

// set up for bluetooth microphone input
UInt32 allowBluetoothInput = 1;
OSStatus stat = AudioSessionSetProperty (
                         kAudioSessionProperty_OverrideCategoryEnableBluetoothInput,
                         sizeof (allowBluetoothInput),
                         &allowBluetoothInput
                        );
NSLog(@"status = %x", stat);    // problem if this is not zero

// check the audio route
UInt32 size = sizeof(CFStringRef);
CFStringRef route;
OSStatus result = AudioSessionGetProperty(kAudioSessionProperty_AudioRoute, &size, &route);
NSLog(@"route = %@", route);    
// if bluetooth headset connected, should be "HeadsetBT"
// if not connected, will be "ReceiverAndMicrophone"

// now, play a quick sound we put in the bundle (bomb.wav)
CFBundleRef mainBundle = CFBundleGetMainBundle();
CFURLRef        soundFileURLRef;
SystemSoundID   soundFileObject;
soundFileURLRef  = CFBundleCopyResourceURL (mainBundle,CFSTR ("bomb"),CFSTR ("wav"),NULL);
AudioServicesCreateSystemSoundID (soundFileURLRef,&soundFileObject);
AudioServicesPlaySystemSound (soundFileObject);     // should play into headset

Hope that helps!

I was able to get this working, but it took some doing. I've pieced together the pertinent piece of code here:

[audioSession setCategory:AVAudioSessionCategoryPlayAndRecord error:&err];

UInt32 allowBluetoothInput = 1;
AudioSessionSetProperty(
    kAudioSessionProperty_OverrideCategoryEnableBluetoothInput,
    sizeof (allowBluetoothInput),
    &allowBluetoothInput);

It is also possible to which sources are available and switch between bluetooth, a headset, handset or the speakerphone but things become very involved at that point. The audio source manager I ultimately wrote was over 700 lines.