Game exports out of Unity with no Error, and then within X-Code it "loads" but only goes to the Unity splash screen and then stops. I have a feeling this is something really stupid on my part...
Check the XCode console. If you've built it for Development it will log handy info there (along with your Debug.Log and Debug.LogError too). Usually a SIG error and/or null reference.
I don't know if there is a better way (I've not had to debug since 3.1 came out), but what I did in those cases was put a lot of Debug.Log statements around my code to see what does and does not execute, and try to narrow it down like that. Tedious to be sure. Mono might help with that more these days, not sure.