There have been a couple of posts on this topic. I have it working and posted an entire write up with screenshots here. http://affiliatesupportforum.com/category/using-wp-3-0-multisite-to-build-nflb-subdomains
With this I am able to build subdomains in less than 5 minutes. No joke, this is seriously fast. It does require advanced technical skill and I have only tested it on my Hostgator reseller account.