That's pretty much the idea. There will be only one external API that developers can use, but internally, we will make sure to make it work with as many payment gateways as possible. We will then implement other features but that should be our core service. I've had to implement many payment processors for different clients in the past years and it really is a pain. I'm basically building this for myself :)
I'm building something similar for myself at the moment as well, so when I saw your product on the front page of HN, I thought "wow, great timing." But looks like it's still conceptual (to everyone else.. I'm sure you've got code in the works already) ;)
So how are you going to be testing the payment gateway APIs that you choose to include? Live accounts or maybe you've got someone at each gateway who can set you up with a test account?
Also - a lot of gateways do include load balancing, so some people might be better off just going with their original gateway, instead of adding another layer of complexity (and costs).
But I definitly think you got a viable idea going here... there is money to be made, simply cause your startup is in an area which wouldn't exist if there wasn't any money being moved around.
