Certification are certainly not required to be a REVA, but knowledge and experience in real estate is a major requirement.
Also, real estate licenses are not required to do most all the normal tasks we REVAs do. Examples of things you
can not do without a license are: showing homes, explain or interpret the legal documents, divulge property info to the public, give real estate advice, handle negotiations, or cold-calling (telemarketing) to name a few. But things
you can do are: create property marketing and agent agent marketing, place advertising, update websites, organize transaction paperwork, deliver documents, set appointments, and more general admin stuff. All of this varies by state of course, but they are pretty similar across the board.
Here's a great list that has the some of the different rules by state -
http://www.realtor.org/toolkits/assistants03d