Well, I guess I wasn’t paying close attention to the feedback on Tim Bray’s counting RSS subscribers - Greg Reinacker describes a method for automatically assigning RSS user IDs (and Derek Scruggs’ work on counting RSS subscribers). I have my misgivings, but it looks promising.
Derek has concerns about aggregator support for HTTP 301 redirects and ETag headers. I’d like to add aggregator support for user IDs to the list. It’s the wild west in RSS and aggregators right now and people will be hopping around for a while. So we need to easily transfer user IDs from aggregator to aggregator. And we need to balance that against the ability to refer others to RSS feeds without piggy backing on existing user IDs.
Plus, I think that people will always want a safety net of anonymity. So it might be nice to reserve a standard block of IDs for that purpose - an aggregator could then be configured to randomly use an ID from that block for a given feed.
I still think that email marketing has a lot of life left in it. But maybe RSS is ready to take on more of the load than I had thought.