I absolutely agree as far as Google Adwords certification and recommend reading this post for more information on those:
- How to Become a Google Authorized Adwords, Analytics or Website Optimiser Consultant by Tom Critchlow
I don't tend to think of the Google certifications as SEO, strictly speaking, but for overall inbound / online marketing, they're absolutely respected, at least among other practitioners.
InboundMarketing.com is the closest thing to SEO certification that I have seen that I consider respectable. The courses they post are created by a number of well-respected authorities. Their FAQ says all courses are free, I assume that includes the exam as well.
