> A big perpetrator here are various strands
> of (Post-)Operaismo that argue that housework
> is "productive labour" in a Marxian sense. No,
> it's not, since Marx's definition of productive
> labour is by definition labour that sold in the
> form of a commodity (or more correctly, labour
> that has been performed after labour-power has
> been sold as a commodity) to a capitalist.
What is productive from the viewpoint of capital is not necessarily what is productive from the viewpoint of working people under capitalism -- let alone from the viewpoint of free associated producers. Furthermore, what is productive for individual capitals is not necessarily what is productive for capital as a whole.
Of course, the labor dedicated to the domestic production of use values is hugely productive for capital as a whole. Capital as a whole presupposes the reproduction of labor power as a commodity, something that would be impossible without domestic labor under any historical capitalist society. If domestic labor contributes to keep the value of labor power low, then capital as a whole benefits from it. In other words, capital appropriates the fruits of domestic labor free of charge. This is a form of exploitation, even if it is not the specifically capitalist form of exploitation -- the production and appropriation of surplus value.
In Marx's discussion of productive labor (Capital, Grundrisse, Surplus Value Theories), Marx is arguing with the classical political economists and keeping the discussion at a fairly high level of abstraction. Mostly, he is concerned with the social relations of capitalist production in general, which goes a long way towards understanding concrete, messy capitalist societies, but doesn't provide an understanding of the latter as a totality.
In that discussion, the assumption is that working people play along -- that they are fragmented and plaint. However, in actual capitalist societies, workers resist and fight in different ways, at different levels of intensity, organization, etc., and the outcomes of those struggles differ. So, in actual capitalist societies, what is "productive" is not just what capital deems productive. What is "productive" is subject to dispute.
I wrote about my interpretation of Marx's views on "productive labor" here:
http://mailman.lbo-talk.org/2006/2006-January/001158.html