Italian radicals are the embodiment of a current of thought inspired from XIXth century liberalism. They mostly seek to emancipate the Italian society from the Church and aim to establish a secular democracy. Their main focus is on social issues, supporting gay mariage/civil unions, and so forth (they played also an important role in the legalization of divorce in the 1970s). They lean left on economic issues (actually, they were associated with a small social-democratic party until recently) and most of them have now joined the PD.
American libertarians are mostly anarcho-capitalists who seek to eliminate every form of State intervention on economy. Their main struggles are on the economic domain (enormous spending/tax cuts, ending federal programs, etc...). Their rhetoric is usually populist (the "little guys" vs the "evil big government"). While they are theoretically on the left for social issues, those issues always end up getting skipped because fighting "big guv'ment" is the most important thing. Most of them would easily vote for religious nutjob who is fiscally conservative over a social progressive who is also a "tax n spend librul".
Oh yes, I know, I am a member of that small socialdemocratic party
I can however assure you that italian radicals, in the last part of the '80s have moved from their social-liberal heritage to a very libertarian, a la american sense, path. Consider their former members Daniele Capezzone and Benedetto della Vedova, for instance, or one of their current leaders, Marco Cappato ... they were among the first and foremost supporters of Pinera-like pension reform !
