As usual Nokogiri was impossible to install. I had to step back to my old MBP because my MBP retina needed to go in for a panel replacement. Updated MacVim, needed the excellent Command-T plugin. Command-T relies on Nokogiri which instantly means the authors of Command-T hate the human race; regardless this plugin is required for proper vim operation.

Tried all manner of... (yes these numbers are current if you brew install the requisite libraries as of January 15th 2013ish):

$ gem install nokogiri -- \  
  --with-xml2-dir=/usr/local/Cellar/libxml2/2.8.0 \
  --with-xslt-dir=/usr/local/Cellar/libxslt/1.1.26 \

But really what was actually missing was gcc-4.2, no longer shipped as part of Xcode. So once I did:

$ brew tap homebrew/dupes
$ brew install apple-gcc42
$ sudo ln -s /usr/local/bin/gcc-4.2 /usr/bin/gcc-4.2

And ran my gem install described above:

Building native extensions.  This could take a while...  
Successfully installed nokogiri-1.5.6  
1 gem installed


Thanks Carina C. Zora for making vim usable on my substitute MBP for a few days!