I'm working for a web company that's trying to monetize our web based email client with display ads (not context based ads i.e. Google Adsense). I can find the pages where advertisers sign up for Yahoo and Google (http://www.google.com/adwords/displaynetwork/ and http://advertising.yahoo.com/adsolution#product=DisplaySolutions), but I can't find where publishers apply for these networks. I understand that you have to have a pretty successful website to join them. Our email client hasn't launched yet, but we have backing from some pretty successful companies and a lot of money to spend on advertising. We would like to have ads in place before we launch. I've emailed the advertising support at both networks, but have received no response as to where publishers can apply. Does anyone know anything about either Yahoo or Google's display ad publishing networks?