indyank - 4:22 pm on Mar 16, 2011 (gmt 0)
the same thing did happen to our wordpress site because of using the simple tags plugin. The new wordress release had somehow broken the plugin which in turn broke our category navigation and we didn't even notice it until march 3. A big mistake on our part for not testing the changes thoroughly. All the category pages resolved to the home page creating some sort of infinite url space and duplicate content.
we don't index those category pages but we do follow the links in them.
since this happened at the same time as the panda update, it made our situation somewhat complex.