So I'm here, half way through GSOC 2012. At this point the Payment module architecture is quite solid and tested well. PayPal payment is already finished and works well with PayPal Sandbox (I haven't tested it on a live site though). I have also set up some unit testing to help test PayPal payment in the sandbox environment.
I'm currently looking at how to implement mock gateways for unit testing, which is the last major element of the project. The mock gateways serve the purpose of mocking HTTP responses from the gateways. Developers can use them to quickly test out their gateway response handlers.
Overall, I'm quite happy with where things are at the moment. There are still a lot of improvements to the architecture as well as PayPal sub-module, but generally, other developers can start using the current architecture to implement other gateways of their choices. There's a simple guide on how to do that here.