John Gruber is skeptical:

Sure, that’s the explanation — not that demand for Windows PCs is drying up. I’m surprised Nvidia couldn’t make up the difference with Tegra 2 chipsets that powering all those best sellers on the “non-iPad tablets” list.

Skepticism is understandable, but I actually suspect that Thailand’s flooding is the explanation. The PC hardware ecosystem is tightly coupled; is it so hard to believe that massive shortages in supply for any key component could lead to decreased demand across the ecosystem? It will probably take several more quarters before anyone can say for sure.