I just upgraded to FSL version 5 on my MacBook Pro running Mountain Lion. The installation was straightforward using the new install script from FMRIB: http://fsl.fmrib.ox.ac.uk/fsl/fslwiki/FslInstallation
Download the python script and then in Terminal navigate to the download location and run: python fslinstaller.py
That will download the files and then install FSL with minimal user interaction (you can specify an installation path – on a single user computer it’s fine to use the default path, on shared computers, I find a better install path is something like /usr/share).
If you are running OS X 10.8 (Mountain Lion), you need to install XQuartz (you need an X11 environment regardless of the version of OS X you are running): http://xquartz.macosforge.org/landing/. This version of XQuartz works with OS X 10.6 and later (and is what you have to install with Mountain Lion).
Now everything should be set up to run FSL 5 on a Mac running OS X 10.8. I’ve not run into any problems so far. Thanks to FMRIB, installation of FSL is now easier than it already was.