I have a subdomain that i use for advertisements. It has openx installed to serve the ads. How can I ensure that everything from the subdomain gets treated like adding rel=nofollow as I don't want Google and other search engines to think that we think the ads are worth following.
your are looking for the X-Robots-Tag HTTP header
in the .htaccess of the server of your subdomain
Header set X-Robots-Tag "nofollow"
see google http://code.google.com/web/controlcrawlindex/docs/robots_meta_tag.html
seems to be supported by bing, too http://www.bing.com/community/site_blogs/b/webmaster/archive/2009/08/21/prevent-a-bot-from-getting-lost-in-space-sem-101.aspx