app/views/tag/index.xml.erb
<?xml version="1.0" encoding="UTF-8" ?>
<rss version="2.0">
<channel>
<% @posts.each do |p| %>
<item>
<title><%= p.post_title %></title>
<link><%= root_url(:only_path => false) %>post/<%= p.post_slug %></link>
<% begin %>
<description><%= p.post_body.slice(0..(p.post_body.index('<MORE>'))).tr("<", "") %> </description>
<% rescue %>
<%= p.post_body %></description>
<% end %>
</item>
<% end %>
</channel>
</rss>