"My guess is that federation across tag spaces will be accomplished by aggregators and search engines. When the subject is avian flu, they'll enable you to compare the resources cited by nonspecialists with those cited by various kinds of specialists (...
Reading lists are OPML documents that point to RSS feeds, like most of the OPML documents you find, but instead of subscribing to each feed in the document, the reader or aggregator subscribes to the OPML document itself.