routes.rb

Myapp::Application.routes.draw do
  
  # signup.host.com
  constraints :subdomain => "signup" do
    resources :accounts
    match "signup" => "accounts#new"
    root :to => "accounts#new"
  end
  
  # Catch all subdomains except "signup.host.com"
  constraints :subdomain => /^(?!signup\b)(\w+)/ do
     root :to => "dashboard#show"
  end
end