Unfortunately this collaboration doesn't exist in the blogosphere where tagging is limited to blog authors. We have added collaborative tagging to some intranet blogs and the results are interesting, but this would really bloom on the world wild web.
Technorati probably has the technology and interest in developing such a system. It could be linked to single blog posts with some simple javascript calls just like the new tracking widget does. Added tags would end up living on Technorati servers and not on mine, but it's something I could probably live with, especially if Technorati would give me back a map of my tags in some standard format (once upon a time we used OPML for something similar).
Tags: Blog, David Weinberger, folksonomy, Technorati