application_controller.rb

class ApplicationController < ActionController::Base
  protect_from_forgery
  filter_parameter_logging :password
  
  private
    def set_current_account
      @current_account = Account.find_by_subdomain!(request.subdomains.first)
    end
end

dashboard_controller.rb

class DashboardController < ApplicationController
  before_filter :set_current_account
  
  def index
  end
end

Gemfile

# Edit this Gemfile to bundle your application's dependencies.

## Bundle edge rails:
gem "rails", :git => "git://github.com/rails/rails.git"

## Bundle the gems you use:
# gem "bj"
# gem "hpricot", "0.6"
# gem "sqlite3-ruby", :require_as => "sqlite3"
# gem "aws-s3", :require_as => "aws/s3"

## Bundle gems used only in certain environments:
# gem "rspec", :only => :test
# only :test do
#   gem "webrat"
# end

routes.rb

RoutingTest::Application.routes.draw do |map|

  # IMPORTANT: this setup assumes you route all *.myapp.com to localhost:3000 or 
  # something similar. This can be done by defining vhost in Apache or creating a PAC
  # file for your local browser.

  # This constraint captures all requests to hosts starting with www
  # You could probably make this match the empty subdomain (myapp.local) but that 
  # means tightly coupling your hostname to your Rails routing configuration like this
  #
  # constraints(:host => "(^www\.myapp\.com)|(^myapp\.com)")
  #
  # which is IMO a very bad idea
  #
  constraints(:host => "^www\.") do
    root :to => 'welcome#index'
    resources :accounts # since accounts are not supposed to be scoped
  end
  
  # This is the default route when another subdomain is supplied
  root :to => 'dashboard#index'
end

schema.rb

ActiveRecord::Schema.define(:version => 20100112224218) do

  create_table "accounts", :force => true do |t|
    t.string   "subdomain"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

end

welcome_controller.rb

class WelcomeController < ApplicationController
  def index
  end

end