I just got this same error when using ZBarSDK, a bar scanning library for iOS. It runs fine when I upload it to my development device or test it on the iOS6 simulator. But when I try to run it for Distribution it fails with:
(null): File is universal (3 slices) but does not contain a(n) armv7s slice: /Users/quique123/Documents/iphone apps/ScanThis/ZBarSDK/libzbar.a for architecture armv7s
where libzbar.a is the library from the sdk.
Any ideas?
Simply remove the bad architecture from Valid Architectures in Target Aguilar Settings and you're good to go :)