Is an iPad a portable media device? A tablet computer? Something entirely "post-PC"? The answer, of course, is "it depends who you ask."
Let's go ahead and define the parent category for iPads (and other tablets like the Kindle Fire, Samsung Galaxy Tab and Google Nexus) as "mobile screens that run a touch-centric OS, don't ship with a hardware keyboard, are larger than a mainstream smartphone and don't offer telephony as a core feature." Call that a tablet, or a "pad" if you must to distinguish it from legacy tablet PCs and the potential fridge toasters of Microsoft's Surface devices.
If that's the bucket we're talking about, some reasonable sales comparisons are available. The four
