View helper I use in my Merb blog's atom feed to create a TagURI.

gistfile1.rb

# Create a Tag URI for the given +article+
# (See http://www.taguri.org/)
# Should return something along the lines of:
#   tag:example.org,2008-12-01:/articles/1
def tag_uri(article)
  tag = absolute_url(:article, :id => article)
  tag.gsub!(/^http:\/\//, 'tag:')
  tag.gsub!('#', '/')
  pub_date = article.created_at.strftime("%Y-%m-%d")
  tag.gsub!(/#{request.host}/, "#{request.host},#{pub_date}:")
  tag
end