routes.rb

  # USER.boldfaced.net
  constraints(Subdomain) do
    get 'work/:id' => 'work_items#show', as: :user_work
    get '' => 'users#profile', as: :user_profile
  end

subdomain.rb

class Subdomain
  def self.matches?(request)
    case request.subdomain
    when 'www', '', nil
      false
    else
      true
    end
  end
end