> The libertarian conception is that groups of people can form hierarchical corporations that compete directly with individuals in the marketplace.

I think a principled libertarian would say that a corporation is nothing but a set of individuals who are working towards the same ends ;)

> It depends on the anarchist what that means in practice.

Does it ever. The gap between a social anarchist and an individualistic one is just as large as the gap between a socialist and a capitalist. Or at least, people argue as if it is :P

> which is also what you get from an unregulated market of corporations

A right leaning libertarian would argue that actual monopolies are rare and short lived, and can only be sustained by something like a state which can prevent competitors from entering the market and otherwise provide support through laws like copyright.

> It's incredibly rare to hear copyright's role in our society even described, let alone criticized; even though that role is incredibly significant.

Yep. It's one of the foundational pillars of our economy.