With the Google Maps Android API v2 you can use a GoogleMap.OnCameraChangeListener like this:
mMap.setOnCameraChangeListener(new OnCameraChangeListener() {
private float currentZoom = -1;
@Override
public void onCameraChange(CameraPosition pos) {
if (pos.zoom != currentZoom){
currentZoom = pos.zoom;
// do you action here
}
}
});