The Search Engine Roundtable reports that it seems to be no longer possible to sign up for the Ambassador Program. Although there is no specific confirmation from Yahoo on that on the Ambassador program page, when one tries to sign up via an affiliate site, we get a notice that reads:

Yahoo will no longer be accepting new Ambassador applications until further notice.

Danny has also heard from a source that Yahoo will be dropping the Ambassador program soon. I will try to get confirmation from Yahoo on this rumor.