sudo gem list | cut -d" " -f1 > gem_list.txt
cat gem_list.txt | xargs sudo gem uninstall -aIx
cat gem_list.txt | xargs sudo gem install
This is useful if you ever want to just reset your gems and clean them up. Or if you upgrade to Snow Leopard and want to make sure all your native gems are in decent shape (since Snow Leopard is 64-bit).