To add another implementation to the pile:

- (MKMapRect)MKMapRectForCoordinateRegion:(MKCoordinateRegion)region
{
    MKMapPoint a = MKMapPointForCoordinate(CLLocationCoordinate2DMake(
        region.center.latitude + region.span.latitudeDelta / 2, 
        region.center.longitude - region.span.longitudeDelta / 2));
    MKMapPoint b = MKMapPointForCoordinate(CLLocationCoordinate2DMake(
        region.center.latitude - region.span.latitudeDelta / 2, 
        region.center.longitude + region.span.longitudeDelta / 2));
    return MKMapRectMake(MIN(a.x,b.x), MIN(a.y,b.y), ABS(a.x-b.x), ABS(a.y-b.y));
}

NB: There are many ways to convert between MKMapRect and MKCoordinateRegion. This one certainly is not the exact inverse of MKCoordinateRegionMakeWithDistance(), but approximates it fairly well. So, be careful converting back and forth, because information can be lost.