It won't happen anytime soon, in my opinion. Their are currently two major exams, WREB and ADEX. The ADEX exam is basically NERB and CRDTS. Something like 38 states take WREB and 43 or so take the ADEX exam. That leaves a few states that take their own state exams.
Having a national exam, and one where students didn't have to have everything riding on patients showing up, would be ideal. I think it's one of the biggest issues that the ASDA needs to continue to fight for.