I solved the same problem I had within my App using xCode 4.2 the following way:
I had a mistake in the declaration in the p.list file, now it looks like that (iPhone App only):
UILaunchImageFile
launch.png
I named the two files launch.png and launch@2x.png. Please note: Apples doc say that the file name extension should not be included, but with me it worked only with the file name extension.
Important: only after the launch images are shown in the summary of the project (click on the project itself in the project explorer and in tab summary, scroll down and see whether your launch images are mapped and shown correctly) I could eliminate this "bug". If the plist declaration is consistent but the images still not shown, drag and drop them into the according fields from the project explorer.
If xCode generates the two files after dropping them in the according fields newly, you are still having inconsistencies in your plist-declaration or file names. You will then see what naming scheme is expected according to the new generated file names of the same two images.
After that the launch images were shown correctly in the summary tab the problem with the cropped launch images was solved.
Last but not least ensure the correct sizes of your launch images, e.g. 320x480 and 640x960.
Good luck.