gistfile1.rb

class UsernameConstraint
  def matches?(request)
    if /^\/(?<username>[a-zA-Z0-9-]+)/ =~ request.path
      User.exists?(:conditions => {:username => username})
    end
  end
end

#...

constraints UsernameConstraint.new do
  scope '/:username' do
    get '/' => 'users#show', :as => 'user_home'
    resources :posts, :as => 'user_posts'
  end
end