Programmatically open Maps app in iOS 6

The best way to do it is to call new iOS 6 method on MKMapItem openInMapsWithLaunchOptions:launchOptions

Example:

CLLocationCoordinate2D endingCoord = CLLocationCoordinate2DMake(40.446947, -102.047607);
MKPlacemark *endLocation = [[MKPlacemark alloc] initWithCoordinate:endingCoord addressDictionary:nil];
MKMapItem *endingItem = [[MKMapItem alloc] initWithPlacemark:endLocation];

NSMutableDictionary *launchOptions = [[NSMutableDictionary alloc] init];
[launchOptions setObject:MKLaunchOptionsDirectionsModeDriving forKey:MKLaunchOptionsDirectionsModeKey];

[endingItem openInMapsWithLaunchOptions:launchOptions];

This will start the navigation for driving from the current location.

Found the answer to my own question. Apple documents its maps URL format here. It looks like you can essentially replace maps.google.com with maps.apple.com.

Update: It turns out that the same is true in MobileSafari on iOS 6; tapping a link to http://maps.apple.com/?q=... opens the Maps app with that search, the same way http://maps.google.com/?q=... did on previous versions. This works and is documented in the page linked above.

UPDATE: This answers my question relating to the URL format. But nevan king's answer here (see below) is an excellent summary of the actual Maps API.

Here's the official Apple way:

// Check for iOS 6
Class mapItemClass = [MKMapItem class];
if (mapItemClass && [mapItemClass respondsToSelector:@selector(openMapsWithItems:launchOptions:)]) 
{
    // Create an MKMapItem to pass to the Maps app
    CLLocationCoordinate2D coordinate = 
                CLLocationCoordinate2DMake(16.775, -3.009);
    MKPlacemark *placemark = [[MKPlacemark alloc] initWithCoordinate:coordinate 
                                            addressDictionary:nil];
    MKMapItem *mapItem = [[MKMapItem alloc] initWithPlacemark:placemark];
    [mapItem setName:@"My Place"];
    // Pass the map item to the Maps app
    [mapItem openInMapsWithLaunchOptions:nil];
}

If you want to get driving or walking instructions to the location, you can include a mapItemForCurrentLocation with the MKMapItem in the array in +openMapsWithItems:launchOptions:, and set the launch options appropriately.

// Check for iOS 6
Class mapItemClass = [MKMapItem class];
if (mapItemClass && [mapItemClass respondsToSelector:@selector(openMapsWithItems:launchOptions:)]) 
{
    // Create an MKMapItem to pass to the Maps app
    CLLocationCoordinate2D coordinate = 
                CLLocationCoordinate2DMake(16.775, -3.009);
    MKPlacemark *placemark = [[MKPlacemark alloc] initWithCoordinate:coordinate 
                                            addressDictionary:nil];
    MKMapItem *mapItem = [[MKMapItem alloc] initWithPlacemark:placemark];
    [mapItem setName:@"My Place"];

    // Set the directions mode to "Walking"
    // Can use MKLaunchOptionsDirectionsModeDriving instead
    NSDictionary *launchOptions = @{MKLaunchOptionsDirectionsModeKey : MKLaunchOptionsDirectionsModeWalking};
    // Get the "Current User Location" MKMapItem
    MKMapItem *currentLocationMapItem = [MKMapItem mapItemForCurrentLocation];
    // Pass the current location and destination map items to the Maps app
    // Set the direction mode in the launchOptions dictionary
    [MKMapItem openMapsWithItems:@[currentLocationMapItem, mapItem] 
                    launchOptions:launchOptions];
}

You can preserve your original iOS 5 and lower code in an else statement after that if. Note that if you reverse the order of items in the openMapsWithItems: array, you'll get directions from the coordinate to your current location. You could probably use it to get directions between any two locations by passing a constructed MKMapItem instead of the current location map item. I haven't tried that.

Finally, if you have an address (as a string) that you want directions to, use the geocoder to create an MKPlacemark, by way of CLPlacemark.

// Check for iOS 6
Class mapItemClass = [MKMapItem class];
if (mapItemClass && [mapItemClass respondsToSelector:@selector(openMapsWithItems:launchOptions:)])
{
    CLGeocoder *geocoder = [[CLGeocoder alloc] init];
    [geocoder geocodeAddressString:@"Piccadilly Circus, London, UK" 
        completionHandler:^(NSArray *placemarks, NSError *error) {

        // Convert the CLPlacemark to an MKPlacemark
        // Note: There's no error checking for a failed geocode
        CLPlacemark *geocodedPlacemark = [placemarks objectAtIndex:0];
        MKPlacemark *placemark = [[MKPlacemark alloc]
                                  initWithCoordinate:geocodedPlacemark.location.coordinate
                                  addressDictionary:geocodedPlacemark.addressDictionary];

        // Create a map item for the geocoded address to pass to Maps app
        MKMapItem *mapItem = [[MKMapItem alloc] initWithPlacemark:placemark];
        [mapItem setName:geocodedPlacemark.name];

        // Set the directions mode to "Driving"
        // Can use MKLaunchOptionsDirectionsModeWalking instead
        NSDictionary *launchOptions = @{MKLaunchOptionsDirectionsModeKey : MKLaunchOptionsDirectionsModeDriving};

        // Get the "Current User Location" MKMapItem
        MKMapItem *currentLocationMapItem = [MKMapItem mapItemForCurrentLocation];

        // Pass the current location and destination map items to the Maps app
        // Set the direction mode in the launchOptions dictionary
        [MKMapItem openMapsWithItems:@[currentLocationMapItem, mapItem] launchOptions:launchOptions];

    }];
}