installing nokogiri gem on snow leopard failed on libxml2 and libxslt checks until I did the following:

cleared out the old libs:

• $ sudo port uninstall -f libxml2
• $ sudo port uninstall -f libxslt

installed the new libs:

• $ sudo port install libxml2
• $ sudo port install libxslt

I was then able to install nokogiri

• $ sudo gem install nokogiri
• Successfully installed nokogiri-1.3.3