I held a technical presentation for Redpill Linpro about running a paywall/metered access with Varnish yesterday.
Slides as PDF are here: RL-paywall-2013-03
Main talking points were how it runs within Varnish, what extra vmods are in use, and finally how it is integrated with your backend/cms.
To keep the VS marketing department happy (Hi!), here is the link to our product page: https://www.varnish-software.com/varnish-paywall :-)