RonWen said:
Jimmy Carter set out to convert the U.S. to the metric system (bringing us up to the rest of the world) but the effort failed miserably.
Probably the Metric Alphabet or the "Decabet" was the final straw.
The history of the metric system is a bit of a hobby of mine. One really interesting book (at least to me) on the topic is "Measuring America" by Andrew Linklater.
There is also this web page on the early history of it:
http://www.metricationmatters.com/who-invented-the-metric-system.html
Few people in America are aware of it but Thomas Jefferson was a big advocate of decimal measurements, and if he had his way the US, not France would have been the first country on a metric style decimal system. Jefferson did get his way with currency, which is why the US was the first country with a decimal currency.
In one sense the US has been fully on the metric system since 1959. That is when all of the old units were redefined in terms of metric units by treaty. For example a yard is officially 0.9144 meters. The US is also a signatory to all of the Metric Treaties, including the first, the "Treaty of the Meter" in 1870.
Under Gerry Ford a metrication act was passed; Jimmy Carter being trained as an engineer realized the utility of it and campaigned for it. This act was later repealed during the Reagan administration due to budget cuts. However it was later reinstated during Reagan and reauthorized again during the Clinton administration. While progress has been slow you can see the signs of it if you look around. Some things in the US are now sold exclusively in metric labelled containers (2L soda anyone?) now, and a lot of containers have both metric and US units.
Here's a list of consumer products now sold in metric sized containers in the US:
http://lamar.colostate.edu/~hillger/products/
The US Fair Labelling and Packaging Act (FLPA) now actually requires both metric and US units on all consumer packages, and 48 states allow metric only packaging. Some industries are lobbying to get the FLPA changed to allow wide use of metric only labeling due to global trade requirements.
Conversion is never easy; it took the French 40 years, Japan took 70 years. Many countries like England and Canada have hybrids.
Construction industries have been the slowest to change. Japan for example still uses traditional units.
http://en.wikipedia.org/wiki/Metrication_in_the_United_States#Metrication_efforts