I also found this incredibly helpful post for those of you who are running cPanel/WHM
http://www.blogopreneur.com/2006/11/...nelwhm-server/
This is for those of you who are going to use the sub.domain.com for new user registrations.
Essentially, it explains about using wildcards in your httpd.conf (*.domain.com) where (*) is the username.
I also should add, hat this is for reseller accounts. I'm on a shared hosting plan myself, so it doesn't help me, but may help someone else.