Have you looked at RegSoft.com? I've seen them for a couple of products.
You could also grow something using PayPal and their API's. I've played a little bit with PayPal's API's for a class project.
I was screwing around with some code a while back and started writing a subscription like service that authenticated against my UBB. Essentially in your customer record, you would need to have some sort of expire date, and if someone renews, you go in and initiate a 1 year extension for the order. I'm guessing that's how Groupee does the member's area.
In my testing stuff, I had a db record that referenced the UBB's User Number and tied their subscription record to that. I never rolled this into my production site because the project died.
I know you are a skilled PHP coder, and it would take you no time to cook up a Member's Area
