It is used in new Xcode (Mac App Store) builds as default compiler.
I'd like to, but there's been some trouble making LLVM/Clang as a cross-compiler on Linux. It's being worked on but not at any quick pace.
I managed to do this (it's actually quite easy): https://github.com/tpoechtrager/osxcross
Maybe time to get started http://lists.cs.uiuc.edu/pipermail/llvmdev/2013-September/065913.html