1. "RE: Why is ideology considered material?" In response to Reply # 0
Yeah, it's essentially Marxist--materialist, to be specific. Ideology can be conceived as being constructed on an economic base, no different in that regard than any other material product of that base. Thus an individual living and working in a given economic system tends to subscribe (or be dominated by), either consciously or otherwise, to the system's dominant ideology.
Terry Eagleton has a nice commentary on Althusser's ideas: "...for him human individuals are the products of many different social determinants, and thus have no essential unity. As far as a science of human societies goes, such individuals can be studied simply as the functions, or effects, of this or that social structure--as occupying a place in a mode of production, as a member of a specific social class, and so on. But of course this is not the way we actually experience ourselves. We tend to see ourselves rather as free, unified, autonomous, self-generating individuals; and unless we did so we would be incapable of playing our parts in social life. For Althusser, what allows us to experience ourselves this way is ideology...
"... I do not feel myself to be a mere function of a social structure which could get along without me, true though this appears when I analyse the situation, but as somebody with a significant relation to to society and the world at large, a relation which gives me enough sense of meaning and value to enable me to act purposefully.
"Most commentators would now agree that Althusser's suggestive essay is seriously flawed. It seems to assume, for example, that ideology is little more than an oppressive force which subjugates us, without allowing sufficient space for the realities of ideological struggle..." (Literary Theory: An Introduction. 2nd ed. 149-150)
You might also look into Raymond Williams' "Marxism and Literature" for a good introduction to the topic, in addition to the Eagleton. And Fredrick Jameson's "Political Unconcious" and "Marxism and Form" explore these ideas in much greater depth.