Here's an interesting chart Apple just threw up at its iPad event today. Apple shipped 15.4M iPads last quarter, more than any PC vendor was able to ship PCs out of their own product lines. Granted the implication here is that PCs and iPads are replacement goods for one another, but I'm not entirely sure that's true. The two categories seem very complementary to me. What does seem to be happening however is tablet purchases like the iPad may extend PC upgrade cycles (e.g. I'll put off a PC upgrade for another year or two and just get a tablet this year), which is reason for concern if you are a PC vendor.