When configuring Mono to use llvm, if you receive the error:

configure: error: Cross compiling is not supported for target i386-apple-darwin11.2.0

Then the llvm-config that is currently the first hit in your path and that is begin picked up by the mono autogen/configure script is does not match the target that you are trying to build in mono.

llvm-config –host-target should help you out and change your path so the correct llvm bin directory is first in the list so when mono’s configure runs llvm-config it gets the correct info.