You can read about it here: http://en.wikipedia.org/wiki/Merchant_Mariner_Credential
As it is still a “proposal” and considering the cost and confusion surrounding the TWIC, I doubt that it will be implemented any time soon. As the economy worsens and a new administration takes over, I believe we are leaving the whole “Homeland Security Era” behind. Or so I choose to believe. The most interesting paragraph of that wikipedia page is as follows:
"The proposal has been criticized by several groups. One organization states that the consolidation reduces the merchant marine officer license, a certificate of professional achievement and status, into a work permit."
Well maybe they could turn out an MMC with a nice design and satisfy them.