First diagnose which version of libxml2 you're using:
bundle exec nokogiri -v
If you have 2.7.3 listed somewhere, you're in bad waters (known to segfault). Install this:
brew update
brew install libxml2
brew install libxslt
Don't miss a single one, or you'll still dynamically link with libxml2 2.7.3, bundled with the OS apparently).
Uninstall the existing nokogiri:
gem uninstall nokogiri
Tweak your building config for nokogiri and rebuild:
bundle config build.nokogiri --with-xml2-include=/usr/local/Cellar/libxml2/2.7.8/include/libxml2 --with-xml2-lib=/usr/local/Cellar/libxml2/2.7.8/lib --with-xslt-dir=/usr/local/Cellar/libxslt/1.1.26 --with-iconv-dir=/usr/local/Cellar/libiconv/1.14
bundle install
Be extra careful for missing - (eg: -with-xml2 instead of --with-xml2...) or bad path (double check these works for you - here with homebrew).
Then you can test again:
bundle exec nokogiri -v
You should see libxml 2.7.8 (not 2.7.3) listed as compiled and loaded.
No more segfaults on either Snow Leopard or Lion for me after that.
Good luck!