For some reason I cant post comments on Gene Callahans blog, so Ill put the comment here. In response to the post where Gene says:
Obligation is the crucial idea denied by libertarian political theory. We can have obligations that we did not agree to take upon ourselves.
I can think of hardly any libertarian political thinkers who say that we have no unchosen obligations. (Rand says it, but that has more to do with her metaethics than her political commitments.) Most libertarians would say that we have a) some enforceable obligations we didnt choose (like the obligations not to kill, steal, assault, etc.), plus b) plenty of moral obligations that arent enforceable.
Of course its true that libertarianism denies the existence of various enforceable obligations that other theories assert; but libertarianism also asserts the existence of enforceable obligations that other theories deny.