JustDave and I are working on a Professional Product - which will integrate with UBB.Threads and offer the ability for users to signup and gain access to forums and groups using PayPal subscriptions.
It will automatically add users to the group when they singup and remove them if they cancel and/or their subscription expires. It'll be a total subscription management system, and with update - other payment services may be added.
After looking into a few of the other payment servies requested over at Infopop community, JustDave has been going back through and changing things a bit so that updates and additional payment services will be easier to add in.
I'm going on a vacation Wednesday for a week - when I get back, we expect to begin live testing at one of my sites. JustDave has been working like a maniac on it lately.
We will have an online demo to look at too - I just don't think our site is "ready for prime time" yet until we hammer out a few more details.
But stay tuned. We will soon have a total solution for offering subscriptions on your site.