It looks like Lion (10.7) includes the latest libiconv. However, if nokogiri gives you guff about having a too-old version of libiconv, you'll have to install a new one.

Rather than compiling from source, just use Homebrew and tap the dupes repo.

brew install libiconv
gem install nokogiri -- --with-iconv-dir=/usr/local/Cellar/libiconv/1.15/

Note that you may have to install and specify custom versions of libxml2, etc.

gem install 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-include=/usr/local/Cellar/libiconv/1.15/include --with-iconv-lib=/usr/local/Cellar/libiconv/1.15/lib