domain.rb

class Domain
  def self.matches?(request)
    request.domain.present? && request.domain != 'example.com'
  end
end

routes.rb

MyApp::Application.routes.draw do

  # domain routes (THIS IS THE IMPORTANT PART)
  constraints(Domain) do
    match '/' => 'user#show', :as => :user_domain

    resources :tasks
    resources :projects

    # any other routes that are domain-specific
  end

  # this needs to be below the "domain" section
  root :to => 'site#index'

end

vhost.conf

<VirtualHost *:80>
  DocumentRoot /path/to/my_app/public

  <Directory /path/to/my_app/public>
    Options FollowSymLinks
    AllowOverride None
    Order allow,deny
    Allow from all
  </Directory>
</VirtualHost>