Well, so long as you realize they'll need to wait for propagation to occur ... what you need to do is get (or create) a client system capable of integrating with your control panel system. If you have CPanel/WHM for instance things like WebHostManager Autopilot, Modernbill, etc start coming to mind. They have features for automatic activation.
Also just keep in mind that this could open you up to more fraud accounts, spammers, etc. And as I said, until their domain resolves to your servers, they'll be able to login to CPanel, but their domain won't show their website until it has propagated.
As Matt said, this can easily be handled by using almost any billing software. However, this would definitely open you up for fraud orders.
To counter this and still use the automated domain registrations/account activation, maybe you can first manually approve an order before it gets setup? (I think ModernBill can handle this, not sure about the others)
This can all be done with Hsphere .. However it is not recommended due to the ebay phishing and other types of unwelcome orders. If you automatically charge credit cards with out knowing what is going on you run the risk of loosing your merchant account due to chargebacks. I have seen this happen it drove the guy right out of business.