We're looking into that at the moment, and have been considering the usual options of DIY, using Recurly/Chargify/similar, going quick and dirty with Paypal/Google, etc. Right now, after a great deal of investigation and contacting many organisations for more details, we are overwhelmingly thinking that a simple, direct system with a payment gateway and merchant account is the way to go. None of the usual recurring billing suspects has even come close to justifying the costs and usually lock-in of integrating their service instead.
However, in the HN goldfish bowl, almost everyone seems to have a different view (though I have noticed a few dissenting voices creeping in recently). Right now, all we've done is a bit of prototyping with the test system for the service we're looking at using and a thorough read of the docs. If we're about to enter a world of pain when we actually want this stuff to work, it would be good to hear any advice about how to minimise the damage and make it through!
Great slides, BTW. Thanks for sharing.