gistfile1.m

// MapViewAnnotation.h
#import <Foundation/Foundation.h>
#import <MapKit/MapKit.h>
@interface MapViewAnnotation : NSObject <MKAnnotation> {
	NSString *title;
	CLLocationCoordinate2D coordinate;   
}
@property (nonatomic, copy) NSString *title;
@property (nonatomic, readonly) CLLocationCoordinate2D coordinate;
- (id)initWithTitle:(NSString *)ttl andCoordinate:(CLLocationCoordinate2D)c2d;
@end


// MapViewAnnotation.m
#import "MapViewAnnotation.h"
@implementation MapViewAnnotation
@synthesize title, coordinate;
- (id)initWithTitle:(NSString *)ttl andCoordinate:(CLLocationCoordinate2D)c2d {
	title = ttl;
	coordinate = c2d;
	return self;
}
@end


// ViewController.m
- (void)drawMarkers {
    NSURL *markers_url = [NSURL URLWithString:@"http://url"];
    NSURLRequest *request = [NSURLRequest requestWithURL:markers_url];
    NSData *response = [NSURLConnection sendSynchronousRequest:request returningResponse:nil error:nil];
    NSError *jsonParsingError = nil;
    NSDictionary *markers = [NSJSONSerialization JSONObjectWithData:response options:0 error:&jsonParsingError];
    for (NSDictionary *marker in markers) {
        CLLocationCoordinate2D tornadoLocation;
        NSString *latitude = [marker objectForKey:@"lat"];
        NSString *longitude = [marker objectForKey:@"lng"];
        NSLog(@"Downloaded lat, lng: %@, %@", latitude, longitude);
        tornadoLocation.latitude = [latitude doubleValue];
        tornadoLocation.longitude = [longitude doubleValue];
        MapViewAnnotation *annotation = [[MapViewAnnotation alloc] initWithTitle:@"Mark" andCoordinate:tornadoLocation];
        [mapView addAnnotation:annotation];
    }
}



// EXAMPLE LOG:
2012-03-23 22:53:53.109 MyApp[13252:11603] Downloaded lat, lng: 39.028171999999998, -84.724039000000005
2012-03-23 22:53:53.109 MyApp[13252:11603] Downloaded lat, lng: 39.825825000000002, -86.238016999999999