check if a CLLocationCoordinate2D is inside MKCoordinateRegion. this is a simplified version of http://stackoverflow.com/a/10574327/368959

gistfile1.m

-(BOOL)isCoordinate:(CLLocationCoordinate2D)coordinate insideRegion:(MKCoordinateRegion)region {
    CLLocationCoordinate2D center   = region.center;
    CLLocationCoordinate2D northWestCorner, southEastCorner;
    
    northWestCorner.latitude  = center.latitude  - (region.span.latitudeDelta  / 2.0);
    northWestCorner.longitude = center.longitude - (region.span.longitudeDelta / 2.0);
    southEastCorner.latitude  = center.latitude  + (region.span.latitudeDelta  / 2.0);
    southEastCorner.longitude = center.longitude + (region.span.longitudeDelta / 2.0);
    
    return(coordinate.latitude  >= northWestCorner.latitude &&
           coordinate.latitude  <= southEastCorner.latitude &&
           coordinate.longitude >= northWestCorner.longitude &&
           coordinate.longitude <= southEastCorner.longitude
           );
}

See Owen Godfrey's later comment (Oct 22) in this same stackoverflow thread:
"The accepted answer is a little verbose, and fails near the international dateline. "
He proposes an alternative in both Swift and Objective-C. See this at the (current) end of the thread.