application.html.haml

!!! 5
%html
  %head
    %title= 'Zetetic - ' + yield_for(:title, 'Blog')
    %meta{:name => 'description', :content => yield_for(:description, "blarf blarf dorf dorf dorf, hooray it's our blog!")}/
    ...

application_helper.rb

module ApplicationHelper
  def yield_for(content_sym, default)
    output = content_for(content_sym)
    output = default if output.blank?
    output
  end
end

show.html.haml

- content_for(:title, @article.title )
- content_for(:description,  @article.lede )
= render @article