This apporach should work too, using MKCircleRenderer
:
MKCircleRenderer *circleRenderer = (MKCircleRenderer *)[mapview rendererForOverlay:circleOverlay];
[circleRenderer invalidatePath];
MKMapPoint mapPoint = MKMapPointForCoordinate(tapPoint);
CGPoint circlePoint = [circleRenderer pointForMapPoint:mapPoint];
BOOL mapCoordinateIsInCircle = CGPathContainsPoint(circleRenderer.path, NULL, circlePoint, NO);
if ( mapCoordinateIsInCircle )
{
//do something
}