Further to my earlier posts,  RSS Metrics & Reporting and FeedBurner answers my RSS metrics question ...  

Via Steve Rubel:

"RSS Measurement Carries Benefits and Pitfalls

Forrester analyst Charlene Li breaks down a new service from SyndicateIQ that can track the number of subscribers to an RSS feed. She notes that the service can also track if a user at a specific computer or IP address opens an article within 24 hours, thereby tracking actual daily circulation of an article."