On NetTuts.com today there's a new tutorial showing you how to use the CodeIgniter framework to create Basecamp-style subdomains in your application.

CodeIgniter is a simple and lightweight PHP framework used to create powerful web applications. Today, we are going to do something nifty: we’ll combine user names and subdomains to make for a more cohesive user experience. If you’re a bit confused, this feature allows your users to access their accounts by typing in a custom url, which maps to their user name, such as harbinger.yourapp.com.

Their setup shows the homepage for the application if you go directly to the domain but changes to a login form when you hit the subdomain. There's a bit of DNS configuration involved to get the subdomains working (you'll have to add in a wildcard to catch the subdomains). They show you how to set this up on your localhost if you're working on OSX or Windows. With a few changes to the Apache server for virtual host and a basic install of CodeIgniter, they get started creating the database tables and control panel application.