It would be great to have this implemented https://stripe.com/ .
Update - stripe is currently an option for one time donations.
We have no plans at this time to integrate Stripe. Democracy Engine is a popular choice for many of our customers and is fully integrated with NationBuilder and also provides its own web service for a very reasonable price. See http://nationbuilder.com/how_much_does_nationbuilder_charge_for_donation_processing for more detailed information on payment processors that we support.