Common LISP on iPhone/iOS

It depends on what you mean by calling a CL function, but most likely ECL will be your shortest path. Start here, as there is a patch for ECL to better accommodate iOS: http://funcall.posterous.com/tag/iphone

ECL generates C code, so you should be in safe territory with Apple's shifting policies.

Have you tried MOCL?

According to the website "mocl is a highly optimizing CL implementation, delivering tight native code via LLVM/Clang".

Maybe you can create a dynamic library, though it is designed for use the other way around.