Memberful is a project by Drew Strojny and the folks at The Theme Foundry. It's not on WordPress, but it will have easy integrations built in. The Theme Foundry has been using it for over a year to process payments. Consider Memberful to be like a hosted version of Easy Digital Downloads, meaning they take care of things like SSL for you. It's interesting to see these guys stepping out. If it's anything like their themes, it will be top notch stuff. I'm going to be beta testing, and I'll report with more then.