Although there are a large number of businesses offering SEO certification none of them can be treated as absolute and there is no such thing, yet, as "SEO certified".
Any company claiming to be "SEO Certified" is being slightly misleading. What they are actually saying is "We paid that company over there some money and they told us we are SEO certified". Usually (but not always) there is some sort of exam or proof of practice required.
Search engine optimisation is very fluid in terms of what it involves because the search engine algorithms which define the rankings of sites are always changing. Furthermore no one outside of the search engines actually knows the algorithms for sure.
The upshot is that as SEO cannot be truly defined it is not possible to create a universal "certification" that everyone would agree on.
So put the "SEO certified" bit to one side and look at the company's track record and achievements. These are worth much more than a certificate.
For more on this see our page Choosing a SEO company.
I'm Tim Hill, a Search Engine Optimisation and Online Marketing specialist. I created this site to help others understand that SEO is not a mysterious black art!.
If your a newbie try the Getting Started in SEO page, otherwise feel free to dig around and learn more.