If you include the splash image Default-568h@2x.png, which would be a 640x1136 (your landscape splash image rotated 90 degrees), that's enough for the iPhone 5 to know to fill the whole screen.
However, if you then uploaded the app to iTunesConnect, it would most likely fail. That's because the standard AIR 3.4 ADT.jar file doesn't use iOS SDK 6.0. You can get around that by either using AIR 3.5 (http://www.adobe.com/devnet/air/air-sdk-download.html), or by getting the patched version of the ADT.jar file (see this thread: http://forums.adobe.com/thread/1073656, in particular the "correct" answer near the top).
Once you have built with either AIR 3.5, or used the patched ADT.jar version of AIR 3.4, and you have the Default-568h@2x.png image included, everything should be good.