An iPad is not a PC.
The meaning of words is determined by usage. And no one (aside from maybe a handful of people posting on this and similar threads) thinks that an iPad is a PC.
Steve Jobs sure didn't - otherwise the term "Post-PC era" wouldn't have been used. Or made any sense.
Definitely a good point. But
And of course 95% plus of the population would just be confused if you tried to use the words interchangeably. Which is, again, the best evidence that an iPad is not a PC.
And any functionalist argument in the form of: A PC does X; an iPad does X: therefore an iPad is a PC; is going to fail because it would have to include smartphones, iPods, videogame consoles, palm pilots, and certain highly evolved kitchen appliances. And that's just nonsense, and clearly contrary to how people actually use words.
So what do people mean when they point to something and say "that's a PC"? Probably they just mean that it runs windows, or osx, or linux, or any other full OS. And what do people mean when they call an iPad or a Xoom or a Galaxy Tab a tablet computer? Probably they mean that runs simplified, lower power OS optimized for a tablet.
Terms like "PC" or "Tablet computer" or "Smartphone" are human created definitions describing different categories...and it's often hard - in any area - to tell where the boundaries between categories lie, especially when it comes to describing human created products.
It's its own device, which we might as well call a tablet computer.