One of my sites had a software bug last year where two plugins could not get along nicely -- which resulted in about 100k nearly content-free pages being indexed by Google. These pages still show up in Google Webmaster Tools and that's starting to be annoying. They show up as being linked from search pages -- search pages which themselves no longer exist.

Google isn't being quick about forgetting about these pages, but here's a nifty trick that I am hoping will help. I have added the following code snipper to the <head> section of header.php:

<?php if( is_404() ) { ?>
 <meta name="robots" content="noindex,nofollow" />

<?php } ?>
This automatically adds noindex and nofollow to all of my 404 pages. I'll know in a few weeks whether it does the trick.