Last week, while I was at SES Chicago, Yahoo! purchased Del.icio.us, a very popular tagging solution. There is no doubt in my mind that Yahoo! is serious about tagging and how they will utilize it in Web search. Even Tim Mayer from Yahoo! made a comment during one of the sessions that tagging may be the next link popularity component to hit Web search. Danny Sullivan, I believe, is vocal that tagging is a "step backwards" and will be way to easy to spam - thus making for a bad advancement in the Web search space.
Forum discussion on the acquisition at WebmasterWorld. And a thread just getting started at our new forums on Yahoo and Tagging.