Lets say you have a source coordinate and a destination coordinate inside your iOS application and you want to launch the pair and have directions load in the built in maps application on the device.
Assuming the device has GPS, you can get the user location lat & long using the syntax, *.userLocation.coordinate.latitude and *.userLocation.coordinate.longitude are dot properties in Objective-C, and they can be used as the source lat & long as seen below. Replace the * with the name of the mapView object in your code. You also need the lat & long of where you are going. Then, just build an NSString with the URL in it, and use the sharedApplication method of UIApplication to launch your URL in the built in map application.
NSString *mapURL = [NSString stringWithFormat: @"http://maps.google.com/maps?saddr=%f,%f&daddr=%f,%f", mapView.userLocation.coordinate.latitude, mapView.userLocation.coordinate.longitude, destinationLatitude, destinationLongitude];
[[UIApplication sharedApplication] openURL:[NSURL URLWithString:mapURL]];
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