OPML Reading Lists for Optimized Feed Sync

Greg Reinacker posts about how efficiently he can sync the new version of FeedDemon because it uses the NewsGator API. So instead of checking his 150 feeds individually it just asks the NewsGator API what is new and then downloads what has been updated. This is definitely a step in the right direction - it would be even better if it was based on a more open standard than the Newsgator API. My previous post on Syndicating Ping State using OPML Reading Lists would allow this to be used by all services and aggregators. In a nutshell, the user subscribes to an OPML Reading List from a service (like NewsGator Online or Bloglines) which has not only your subscriptions but an HTTP etag for each subscription. As in Greg's example, when you want to update your feeds, you get the OPML file and compare the etag for each subscription with the etag that the aggregator has. If the etags don't match then something changed and the aggregator needs to get it. This isn't a slam against Newsgator or Greg - they set out to build something that would work today. I just think it would be great if we could make this kind of efficient pinging / subscription sharing work generically.