Tags may not necessarily be a librarians worst nightmare but librarians may be tagging's salvation.
As information professionals, Librarians have long known what many of us are only now discovering - the need for standards.
Let's face it, up until a few years ago (before the ascendency of the Web and more specifically, Web 2.0) most of us didn't need to know about tagging or cataloging. We simply didn't have access to enough personal information such as textbooks, journals, personal references etc., to make tagging or cataloging worthwhile.
The "Read/Write" web has changed all that. Now we are awash with info and tagging is one way to deal with it all.
As long as the tags are not shared - no problem. If the tags I have chosen make sense to me and allow me easy access and retrieval for the information I've found, then I'm good. The wheels fall off the wagon when I share my tags with others via del.icio.us, furl or ... What makes perfect sense to me, might make no sense at all to someone else.
Laugh though you may, I suggest that all taggers need a tutorial on the Dewey Decimal System, Sears Subject Headings, the Library of Congress cataloging system or some other structured subject language or controlled language, otherwise they/we risk creating the tagger's, "Tower of Babble".
My $0.02 worth.