[self.mapView.userLocation addObserver:self  
            forKeyPath:@"location"  
               options:(NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld)

               context:NULL];

-(void)observeValueForKeyPath:(NSString *)keyPath  
                     ofObject:(id)object  
                       change:(NSDictionary *)change  
                      context:(void *)context {  

    if ([self.mapView isUserLocationVisible]) {  
        [self moveOrZoomOrAnythingElse];
        // and of course you can use here old and new location values
    }
}