Face it guys, the Americans have always gone their own way as far as international standards. The rest have PAL, the Americans have NTSC. American dates are back to front. Americans use a mains voltage of 110V, while most of the world is 220-240V. In the International System of Buoyage basically the whole world (Region A) uses one system and the US (plus one or two other countries - Region B) is back to front compared to the rest. Even the ITU has a special notation for telephone national and area codes for North America, which is different from the rest of the world.
When you're as big as the US you can develop your own standards but the problem comes when you have to interact with the rest of the world. For example, I'm sure several deaths have been caused in the Caribbean because many Americans do not know the international marine code flag for Diver Down, since they use a locally developed flag.
One of the other correspondents mentioned that the UK has been successful in its changeover because it took its time in changing. Most Antipodeans I know in the UK consider the changeover in the UK to be a total mess, precisely because the changeover has dragged on. The UK started its conversion before New Zealand yet in New Zealand it was all over in 5 to 8 years. In the UK it's still going on 30 years later.
The London Metro newspaper frequently quotes temperatures in its headlines in Fahrenheit, and all the street markets still sell produce in pounds (although they are forced to also display the prices in kg). Feet, yards and miles are still the norm when talking distance, although most people can deal with cm as well as inches. Milk as well as beer is still sold in pints.
In New Zealand the government got fully behind the conversion, unlike in the UK. They published a series of conversion pamphlets that were free for the taking in all post offices and government buildings. Everyone carried the pamphlets with them for reference and the whole conversion was over and done with in 5 years or so. It wasn't necessary, as one correspondent mentioned, to wait for generations for the old fogeys to die off and the kids who'd learnt metrics from day one at school to grow up. Even septuagenarians survived the conversion in NZ without ill effects.
By the way, I once heard that the US Congress passed a law in, I think, 1861, allowing the Metric System to hold equal place alongside the Imperial System in the US. If so, this must stand as the least successful adoption of the Metric System in history.