I know a lot of people who are very much into tagging--folks who kindly label all their blog posts, tag their photos, tag their video streams, you name it. That makes it super easy when I want to go and looks for tags like Shanghai or eTech.
BUT, how useful is tagging as a personal organizing tool?
Peachy Blog has a post talking about how he finds link retrieval in del.icio.us, a popular tagging tool, "just a bit painful," and says he'd rather look at his tags as a set of social bookmarks (ie my links compared to yours) or a link whitelist (nice term-what does it mean?)
David says, "I'd like some way of filtering pagerank based searches from Google or Yahoo by a set of such trusted links," and I think that is a great idea--but I wonder about two things:

1. How well will tagging work as an organizing and information retrieval method when there are millions of tags?--That's where having additional filters, such as identity, trust or cohort group becomes relevant--becomes needed.

2. How can developers move tagging into a wider market? I describe tagging to non-geek friends and they are interested, but these folks aren't blogging, don't use tag-friendly photo services and are a world away, still--how can the tools bring them closer?