CLLocation *userLoc = [[CLLocation alloc] initWithCoordinate:appDelegate.mapView2.userLocation.coordinate];
CLLocation *poiLoc = [[CLLocation alloc] initWithLatitude: [aPOI.latitude doubleValue] longitude: [aPOI.longitude doubleValue]];
	
double dist = [userLoc getDistanceFrom:poiLoc] / 1000;
	
NSLog(@"%f",dist);

http://www.iphonedevsdk.com/forum/iphone-sdk-development/35043-calculate-distance-between-two-latitudes-longitudes.html

Calculate distance between two latitudes / longitudes

Objective C