I, probably somewhat optimistically, believe that most Americans already know the metric system - if not, I can teach them in about five minutes.
The issue, then, is not knowledge of the system, but usage of the system. Using it entails a difficult period of transition, changing signs, having two sets of tools, speed signs, speedometers, labeling, container sizing, etc, etc, etc. Though, we do have, oh, the records of the transition for every country in entire world (other than two little countries just big enough to fit in your pocket) to guide us, how hard can it be?
I heard once (citation?) that conversion to metric was to be a joint effort between Canada and USA, but didn't quite go as planned - more like:
USA: "Hey dude, we should, like, totally do the metric system!"
Canada: "Metric, eh? I dunno."
USA: "Oh yeah, it'll be the bomb! Everyone else is doing it. We'll be able to, like, ask for the next size larger socket without needing a calculator. It'll totally be like: 'Hey dude, gimme the 10 cuz this 9 is too small' instead of 'Hey, uh, dude gimme the, uh, y'know, the one bigger than this one, I think it's half of something' and you'd say 'yeah, uh, is it an eighth bigger you want or a sixteenth? An eighth? So, that's, uh, two goes into eight four times...so that half is...four over eight, plus the eighth bigger...five eighths?' and I'd say: 'Nah, this one is too big, gimme the smaller one' and you'd say: 'So that's, uh, sixteenths? Uh, eight goes into sixteen twice, so...frack it, where's the hammer?'"
Canada: "Yeah, I know, eh?, that really blows. Let's do it!"
USA: "Rock on! You go first."
Later...
USA: "Gee, looks like you're having trouble with that..."
Canada: "Yeah, way more to converting all our signs and stuff than we really thought...this is getting expensive!"
USA: "Hmm, yeah...good luck with that, see ya!"
Ok, I made all that up, but it was believable

.
Anyway, I found this interesting: "The U.S. adopted the metric system in 1866. What the U.S. has failed to do is to restrict or prohibit the use of traditional units in areas touching the ordinary citizen: construction, real estate transactions, retail trade, and education. The U.S. has not made the crucial transition from "soft metric" to "hard metric", so that "1 pint (473 mL)" becomes "500 mL (1.057 pint)", with the traditional equivalent fading into smaller type sizes and finally disappearing."
link
I really hate that we're the last kid on the block that still can't tie their shoes...
Fun Fact: 1790: Thomas Jefferson proposed a decimal based measurement system for the USA. A subsequent vote in the USA congress to replace the current UK-based system by a metric system was lost by only one vote. (dammit, dammit, dammit!)
I'm not seeing much in the line of an existing movement to get the metric system adopted in America. How do we get the ball rolling?