I wasn't referring to selling them a subdomain, rather letting them just sub with no extra cost purple, like Lesli said, for people who don't want to buy but don't want ad supported hosting either. Themed is an interesting idea-giving a greater degree of personalisation and of control to the client.
I also agree that this wouldn't be a service for businesses, but then I would think that few businesses would be working with a re-seller anyway, most choosing to have their own hosting in some form (unless its because I designed their site of course!).