By default, OS X comes with a very antiquated version of GNU Bison (2.3 on my machine). I've had to install v3 on my machine twice now, so I figured it was something that should be documented - if for no other reason than to help Future Rob.
Go to the Bison page and download the latest version of Bison. Extract the compressed file to a somewhat permanent location on your machine (e.g. /Binaries/bison).
Open a terminal window and navigate to the folder you setup in the download step. The instructions for installation are in the INSTALL file. I was able to build a binary version by running ./configure followed by make. This will output a bison binary in the src folder.
Typing bison in a terminal window will still point to the old version of Bison. I was able to fix this by renaming the old file in /usr/local/bin and setting up a symlink to the new version.
Rename the old version:
mv bison bison-2.3
Setup a symlink to the new version:
ln -s /Binaries/bison-3.0.4/src/bison bison
Annoyingly, OS X checks /usr/bin prior to /usr/local/bin when it looks for binaries. This can be changed by editing /etc/paths and moving /usr/local/bin before /usr/bin. Note that this file will need to be opened with root permissions.