I'm not sure when Apple began manufacturing in China, and it may just be coincidence, but I do know that my older Macs (pre 2001, iBook 500, Ti 500, 3400c) have never had a single hardware malfunction, whilst both of my post-2001 laptops (iBook G3 700, PowerBook 1.67) have had to have logic board replacements, and the 1.67 LCD is starting to have problems.
However, I believe that rather than the manufacturing location, it is likely the increased production volume over the last few years, if anything, that may lead to quality control issues. I may be wrong here, but I think the Chinese plants are simply assembling the final product and have nothing to do with the individual componenets.