Client subdomains and custom domains

[eluser]malzahar[/eluser]
I have a CodeIgniter application that I've been using for clients that has proven to be quite successful. As of right now it is installed individually on each client's own web hosting servers however I'm planning on making this product a hosted service available to the public at a monthly fee. I'm in the process of converting the code and am wondering how to handle subdomains and custom domains for this application. I've never had to do anything like this and was wondering if anyone had in any insight on how this could work with codeigniter.

Basically when a user registers they would have their own subdomain like www.username.myproduct.com. Similarly, clients could use their own domains in place of this method. I know this is a common practice in many web applications but I'm not just not sure where to begin.

Any help would be greatly appreciated.