Improved! A slight alteratiion of DHH's dynamic subdomain technique.

gistfile1.rb

# 1) Point *.example.com in your DNS setup to your server.
#
# 2) Setup an Apache vhost to catch the star pointer:
#
# <VirtualHost *:80>
#   ServerName *.example.com
# </VirtualHost>
#
# 3) Set the current account from the subdomain
class ApplicationController < ActionController::Base
  before_filter :set_current_account
  
  private
    def set_current_account
      # I personally prefer "request.subdomains.last" because I see people enter 
      # "www.subdomain.myapp.com" quite often.
      @current_account = Account.find_by_subdomain(request.subdomains.last)
    end
end

# 4) Tie all top-level requests off the current account
class CustomersController < ApplicationController
  def index
    @customers = @current_account.customers
  end
end