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
Windows version of getting started is available at https://github.com/kripken/emscripten/wiki/Using-Emscripten-from-Visual-Studio-2010
More out-of-date things on this gist is that the required Clang version is now 3.2, not 3.1. Also, one needs 'python2' executable, 'python' is no longer used on OSX.