The problem seems to be that the libSDLmain.a you made is 64 bit, which is done by default if you have Snow Leopard installed. You need to explicitly compile the library for i386 and PowerPC if you plan to make Universal Binaries.
You can have my libSDLmain.a (it has i386, x64_64 and ppc all in one file): Download here
Edit: If you use my build script you might want to change CFBundleIdentifier and CFBundleSignature for your application.
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