Just to clarify. Even assuming that unit labor costs were not to rise as the entire US based industrial reserve army of labor is called into action, capital still confronts a looming labor shortage due to the immensity of the capital that is being accumulated--Greenspan is only being honest here.
This has given a special virulence to the drive to open up abroad zones of exploitation to which constant capital that would otherwise build up as excess capacity due to a domestic labor shortage can be exported and valorized--I think this is the most important context in which the China question should be considered.
Of course the shameless exploitation of human labor there will only result in a squandering of human life that will intensify the shortage of labor.
Now if unit labor costs do rise sharply in the US as reserve army of labor is further exhausted, this will exacerbate the shortage of labor which can be exploited at an adequate rate and only add to the virulence of imperialist politics aimed at overcoming the labor shortage (tight monetary policy is no real solution to this problem; its only aim can be to slow down the rate of accumulation until capital actually solves the problem of making available to itself sufficiently exploitable labor).
I only emphasize again that the effective cause of possible crisis here--as Greenspan clearly recognizes-- is not underconsumption or realization difficulties (Marx was not an underconsumptionist) but difficulties in exploitability in that hidden abode of production.
Yours, Rakesh