Any business that take this issue seriously will settle for good references like organic google ranking. There aren't any impressive certificates out there because this is an ever changing field that is controlled by private companies like google, yahoo, ask, ms.. Results prove you can do it. a cert will just mean you know same facts but not more than that. I work in the field of SEO PPC etc for a long time and I can get almost any job because i can show previous results...
SEO is more of an art than a science. ask any seo consultant out there and you will likely get different answers/approaches on how you can rank high in the SEs. so any certification i believe is going to be useless as there is no absolute truth in seo and its methodologies.