Currently, user profiles are in the format of website.com/username which is fine. However, some heavier users wish to use custom domain names. This is similar to Tumblr. Use
You can use the following examples:
Basic PHP:
How to let PHP to create subdomain automatically for each user?
CodeIgniter:
http://net.tutsplus.com/tutorials/php/basecamp-style-subdomains-with-codeigniter/
Zend Framework:
Query regarding subdomain redirection in Zend Framework
$routeitem = new Zend_Controller_Router_Route_Regex('(.*)',
array(1 => '', 'controller' => 'redir', 'action' => 'view'),
array(1 => 'hash'),
'%s'
);
Drupal:
http://drupal.org/node/146344
You're after wildcard DNS; http://en.wikipedia.org/wiki/Wildcard_DNS_record
Unless you have your own server (as in a dedicated or VPS server where you can configure apache) this won't usually be possible as shared hosts normally won't allow this.
Here is a good guide on how you could set it up.