In a panic after Googlebot got at my Viewgit (my fault really for having no robots.txt!), I frantically hacked the template to add "noindex, nofollow". After it quit, I made it a proper ~20 line plugin that hooks into header. You can give it different attributes in the config file setting $conf['robots']; I added a comment to example-localconfig about it because it seems like a pretty commonly necessary option. The markup still validates.
Use it if you wish, but I needed it and I thought somebody else might.
Shoot me an email if I've formatted this wrong somehow, this is my first patch with Git.