tedster - 4:14 am on Sep 20, 2012 (gmt 0)
Did you check the URLs where Google says they found links to those tag URLs? It's one thing to make the URLs 404 - but if the site still shows the links to visitors who are not logged in, the errors can continue indefinitely.
That said, I'm currently working with a site that has 8,000 indexed URLs. Technical errors over Labor Day generated over 48,000 new 404 errors that are still not fixed. There have been no ranking or traffic problems from this issue so far - not even for newly published pages.