There are a number of additional dependencies required for getting things installed on OS X. Starting with a blank slate OS X machine, this is the process it takes:
# Install Xcode Command Line Tools
# Install Homebrew
ruby -e "$(curl -fsSkL raw.github.com/mxcl/homebrew/go)"
# Some deps
brew install node spidermonkey closure-compiler yuicompressor
brew install llvm --with-clang
cd ~/Repos
git clone git://github.com/kripken/emscripten.git
For everyone being slapped in the face with a version 3.3 of LLVM and Clang, the solution is simple: just manually download and install LLVM and Clang v3.2. For the lazy folks among us, you can just follow this guide: https://gist.github.com/shovon/6151878
Don't worry, this will not override your current installation of LLVM.