With the default security settings on MacOS 10.8, Apple refuses to launch apps that are signed with a signature that is validly signed by Apple's CA if that key is for the iOS store instead of the OSX store.
% codesign -dvv DaliClock.app
DaliClock.app/ Contents/ MacOS/ DaliClock
bundle with Mach-O universal (i386 x86_64)
CodeDirectory v=20100 size=7246 flags=0x0(none) hashes=356+3 location=embedded
iPhone Developer: Jamie Zawinski (Y5M82TL69N)
Apple Worldwide Developer Relations Certification Authority
Apple Root CA
Aug 22, 2012 1:15:13 PM
Sealed Resources rules=4 files=3
Internal requirements count=1 size=184
That right there is a valid, paid-for and un-expired cert, issued by Apple's one-and-only CA. A cert is a cert and a CA is a CA, so the only justification for this behavior is that they want to extort an additional hundred bucks a year out of you if you develop apps for both iOS and OSX.