Americans have happily gotten into numerous subcompacts before, and it will happen again. We have been upsizing our cars though and are in need of shrinkage. Not just from FIAT, but Ford and Toyota are having the same idea by giving us the Fiesta and IQ at about the same time FIAT hits. Evidence comes from Honda who's having no trouble selling the Fit here.
America's a big place, with many different auto markets. Yes, there are places where the pickup will continue to rule (and rightfully so) but there is certainly room for more compacts than we have, and that's where the growth in the American auto market is going to be comming from over the next few years anyway.