I've been using premiumcast.com

for the school of podcasting. It not only allows me to create user specific RSS feeds, it allows me to sell digital downloads, create an auto responder podcast (great for courses), and if someone unsubscribes they lose access to the information. They also have the ability to create a membership site, and turn your wordpress blog into a membership site (all though paypal, and premiumcast ). 

Private Reply to Dave Jackson (new win)