It's a little more than a silver coin.
http://en.wikipedia.org/wiki/Amero
It's some economists play thing. And part of a "Future for america" plan. Not sure how influential that think tank is but its more than just a gimmick.
Offcourse the problem is not so much that it's the amero, its more that it's not the dollar. Even if americans all switched to the euro we'd be one step closer to a global currency.