As of 2017, it’s just brew install r. See @Andrew’s answer below.
As of 2014 (using an Yosemite), the method is the following:
brew tap homebrew/science
brew install Caskroom/cask/xquartz
brew install r
The gcc package (will be installed automatically as a required dependency) in the homebrew/science tap already contains the latest fortran compiler (gfortran), and most of all: the whole package is precompiled so it saves you a lot of compilation time.
This answer will also work for El Capitan and Mac OS Sierra.
In case you don’t have XCode Command Line Tools (CLT), run from terminal:
xcode-select --install