You don't have to "create" subdomains.
It's pretty easy to set up if you have access to the Apache and BIND Configuration.
Took me 4 minutes to get this working
- Setup a "wildcard domain" in BIND
*.yoursite.com IN A 123.123.123.123
- Setup a vHost for *.yourdomain.com
- Place a mod_rewrite or PHP (or Perl, Python, ...) script there to transform the hostname into smth. like http://www.yoursite.com/forum/member.php?username=...
And you're done.