application_controller.rb

class ApplicationController < ActionController::Base
  before_filter :set_current_account

  private

  def set_current_account
    unless current_account
      render :text => "Account doesn't exist", :status => "404 Not Found"
    end
  end

  def account_subdomain
    request.subdomains.first
  end

  def current_account
    return @current_account if defined?(@current_account)
    @current_account = Account.find_by_subdomain(account_subdomain)
  end
end

foo_controller_test.rb

class FooControllerTest < ActionController::TestCase
  fixtures :accounts

  context "A valid account subdomain" do
    setup do
      @request.host = "foobar.domain.com"
    end
    
    should "set an Account instance from the subdomain" do
      get :index
      assert_equal accounts(:foobar), current_account
    end
  end
end