Add Clickbank as a gateway

There are quite a few forum threads with people asking about Clickbank as a gateway for various plugins (Pro Sites, Membership, Marketpress), but I don't see anything in the Features & Feedback area so I thought it would be good to add.

For me personally, I'd like to be able to use Clickbank as a gateway for Pro Sites. I would like to be able to charge an up-front fee and then a monthly fee.