The iPad business being closer to Apple's iPod business than the carrier-entangled iPhone business isn't a new theory, but given the increased maturity of the product line, it's one that's increasingly subject to objective rather than subjective analysis. And that's just what Ben Thompson of Stratechery has started doing, not only for today, but what it could mean for the future:
Today's thin clients, on the hand, specialize. A pure tablet is superior for touch-based applications; a pure PC is superior for keyboard-and-mouse ones. An e-ink reader is superior for reading, and a 13-inch iPad would be superior for (in my case) drawing and making music. And while many people now use two devices, I think that's only the beginning (I'm personally at four and the 13″ iPad would be number five).
iPod begat iPod mini which became iPod nano and, eventually iPod nano and iPod shuffle. The original iPod became iPod classic and gained even more storage, and eventually the iPod touch was introduced. Apple took an expensive, singular MP3 player and grew it to serve a wide range of customers and their needs. Beyond pricing and sales model, iPad could indeed be closer to the iPod business in the same way. iPad begat iPad mini. iPod touch is arguably already iPad nano. What could be next?
Thompson's article is incredibly in-depth. Give it a read.