Since I am now hairless after installing Arachni on OS X, I’ll hopefully save someone else the same ordeal. Various problems with the OS X toolchain not playing nice with the latest Ruby, then nokogiri causing a segfault because of using an old version of libxml2… etc.

Anyway, finally got a working version as follows:

1. Install GCC for Lion: https://github.com/kennethreitz/osx-gcc-installer/downloads
2. Edit ~/.bash_profile and add:
   
export CC=gcc-4.2
export CXX=g++-4.2

3. Install RVM as per these instructions, remember to make the change to ~/.bash_profile as mentioned on the page and the reload the shell.
4. Install Homebrew
5. Install libxml2 and libxslt:
   
brew install libxml2 libxslt

6. Link the newly installed libraries:
   
brew link libxml2 libxslt

7. Install the nokogiri gem:gem install nokogirionce installed, run:nokogiri -vand check that it’s linked to the version of libxml2 you just installed.
8. Install Arachni:gem install arachni
9. Marvel at the lack of segfaults