iphone 5 is 1136x640 (or flipped).
If you have not updated your unity, the old unity will only recognize it as a regular iphone 4s and give your letterboxing around your stuff.
When you have update your unity you can figure out the formfactor by looking at
iPhone.generation == iPhoneGeneration.iPhone5
If your xcode build does not make a Default-h2[somenumbers]@x2.png, it will force letter boxing as well. (If you want to force it, just delete this file if it makes it.
NGUI only has access to the Screen.height and Screen.width that unity provides and that depends on the version of unity.
I hope this helps.