Xcode copies all of your resources to a folder like below when debugging:
~/Library/Application Support/iPhone Simulator/5.1/Applications/{Application_ID}/{ApplicationName.app}/
The resource files seem not updated all the time, i.e. if you replace a resource file in Xcode, the change may not be visible when the app is launched, no matter if you clean and rebuild the whole project. So what I had to do is just to delete that folder and rebuild.
I guess there might be better approach, and I still have no clue of how to find the app id except debugging, or checking the created time of the folders. Any idea?