On the iPhone, the Location manager that provides the GPS location is a nice API to use.
It does have some issues though: CLLocationManger doesn't work if it's called from another thread !
I first noticed something was really funny when my delegate wasn't being called at all.
Neither – locationManager:didUpdateToLocation:fromLocation: nor – locationManager:didFailWithError: was called and my application was just waiting there forever for some GPS information.
My first thought was that it was some issue with my memory management as I wasn't holding a reference to the location manager in any class, just in the method where it was created. But still, it didn't work.
Then, I though it was a problem about the threading model being used (I waited for the GPS location in another thread in order not to block the GUI). Sure enough, that seemed to be the problem, and at least another person complained about it. Not sure it is a matter of threading or a matter of memory pool being used.
But more to the point, always create your CLLocationManager instance in the main thread, and not in another thread. Having a singleton method there which is called from the main thread somewhere assures you that the location manager is created in the proper thread/pool.