Canada. This is kicked around in the news from time to time. I doubt it will happen in the foreseeable future. The amero certainly doesn't have much public support here, at least. Especially with statements like this regarding the Canadian economy:
"For a "middle power" whose economy is weak and exchange rate declining steadily, such a maneuver may be in the best interest."
The author of that gem obviously hasn't been paying attention to things economic lately.