"So why are so many Marxists goldbugs who distrust fiat money as a sham? Isn't it because gold embodies labor time and credit money doesn't? Even if credit money is guaranteed by a state?"
We know that Marx sided not with the Currency but the Banking School, so he did not have an allergy to credit money and would presumably have been sympathetic to post Keynesian ideas about endogenous money. See this interesting paper http://72.14.253.104/search?q=cache:LDrPqr2Unx8J:mercury.soas.ac.uk/economics/workpap/adobe/wp130.pdf+marx+fiat+money+costas&hl=en&ct=clnk&cd=4&gl=us&client=safari
Grounds for fear of abuse of fiat money--inflating away small pensioners, inflationary attack on the real wage, geopolitical tension as a result of competitive devaluation. What Michael Ballard was saying, I think.
While Marx does say that money must be a commodity to serve as a measure of value--and perhaps we should see inflation targeting as a kind of return to commodity money--he also emphasizes that money excludes commodities from themselves instantiating universal abstract labor time: monetary exchange is thus (and paradoxically!) more restricted than generalized barter because only money--not commodities--can buy other commodities. I don't think neo classical economic theory deals with the reason for and consequences of this restriction.
Moreover, I was emphasizing that Marx traced this external contradiction between money and commodities, between the universal and relative value form to the inner contradiction latent within the commodity.
I saw on BookTV that even the right wing libertarian Mark Skousen thinks that Marx's distinction between barter and monetary circulation is fundamental, one of the great economic ideas. Yet he had very little idea of the depth of Marx's thinking. But then so too have most Marxists had little grasp of the profound dialectics at work--excepting of course Evald Ilyenkov who drank himself to death in 1979.
Perhaps Chris Doss knows more about him?
Rakesh