I would agree that it certainly eats in to MacBook and netbook sales. Most of these iPad users most likely own and still use a desktop computer... and a laptop. It certainly appeals to certain types of people but these people are not the same as the desktop users. When I think of "PC users" I think of people who do everything on their desktop because it's better for what they need to do. They buy high-end computers and they buy them less often. These desktop PC users are not going to stop buying desktops because of the iPad. Some of them might get one and carry it around because it's easier than a laptop but they will continue to buy home PCs as well. I also think that many people ran out and bought one just because it was new. Wait for the hype to die down. These sales numbers are consistent with any new idea that Apple puts out. People buy it regardless of if they need or would use it.

Conclusion: The iPad is a secondary device. In the same category as an iPhone, Android or even the new ChromeBooks.