I would imagine that Krugman believes that Samuelson has exposed the logical fallacies in Marx's transformation procedure. William Baumol actually attempted a reply; the debate is summarized in the History of Marxian Economics, vol II.
Brad refers to the empirical refutation of Marx's immiseration theory, though does not specify what he understands by it. I am reminded here of Nathan Rosenberg and Wm Birdzell's absurd summary of Marx's exploitation theory in How the West Grew Rich, though Rosenberg is one of the few bourgeois economists who has actually read Marx carefully (it is thus no surprise that in a time of utter confusion in bourgeois economics, their leading historian of economic thought Mark Blaug would be championing the one among them who has done some serious thinking about Marx--well, in the mid-70s). By the mid 80s, Rosenberg could actually write that real wage increases are a refutation of Marx's positive theory of capital dynamics.
But who other than Marx predicted the trend of real wage increases in the course of accumulation (while showing how accumulation set real parameters on the improvement in moral-historical elements of the wage--see here of course Howard Botwinick's Persistent Inequalities), their cyclical movement due to the cycles of accumulation itself, and the termination of those real wage gains in late stages of accumulation if not (or even if) accompanied by ever greater intensity within the labor process (that is, it is not enough to determine what workers are getting but also to determine what they give to the capitalist in the form of the expenditure of energy)?
In terms of wage theory, at a very high level of abstraction, I know of no more stimulating attempts than Henryk Grossmann's (his 1929 work recently translated in the History of Political Economy) and Paolo Giusanni's brilliant essay in the International Journal of Political Economy from the early 1990s. I do not have the cites with me, but will post them at a later date in a separate post.
>From Bukharin to Wallerstein and Edward Herman today, there is an attempt
to confirm the immiseration theory in the context of world capitalism.
But this work tends to be Lassalean in that the wage is thought to be held
at subsistence level by the existence of a massive reserve army of labor.
At the same it was no refutation in the 1920s to compare British wages to
where they had been in the 1850s since some of the production structure
was now in India and elsewhere, and it is no refutation today if the
performance of any national capital is evaluated through time solely
within fetishized national boundaries.
best, rakesh