When we were initially investigating the space Amazon and PayPal looked to be the only payment processors who supported pledges (as opposed to instantaneous transfers). PayPal actually had some advantages, but they were outweighed by the disadvantage that they have never actually processed our paperwork: http://blog.unglue.it/2012/05/03/unglue-it-payment-options-a...

If there are other processors that support pledges, we aren't aware of them (though we welcome suggestions!).

We're working on an alternative funding model that lets us deal with this, but that's a question of business logic as well as software. Mind you, we have a fair amount of both in place already, but there are a lot of moving pieces. We'll keep people posted via our blog, newsletter, Twitter, & Facebook page.