Mainframes never achieved the dominance of PC's precisely because they weren't as useful; so it is logical to say that PC's will continue to dominate embedded devices because the iPad isn't as useful.
Embedded devices never even had to compete with PC's on units sold. That didn't make the PC an afterthought then, so what's different now?
These padigm shifts aren't about what is inside the device. They are about how the devices are used and how that usage impacts people and society.
The glaring, obvious sign that the world is changing is a grocery store checkout line. Five years ago, a couple people in the store might be looking at BlackBerries. Now, everybody and their kids have a device, and often, those devices are talking to each other, via wifi, 3G or Bluetooth.
That is the people impact in the Post PC world. And that impact will be as large as the impact PCs had on the world (IMO). I just hope it's a positive impact.
I was on my way out of town and stopped at the new supermarket to pick up a sub for dinner. As I was checking out, a young guy and his girlfriend were in line in front of me.
He says to the cashier, "you know your cell phone coverage sucks."
The cashier says, "yea, only Verizon works."
The reply was, "I bet they're sorry the didn't think of that when they were building it."
> everybody and their kids have a device, and often, those devices are talking to each other
The inter-device communications are just part of the app, another way of describing the cloud.
The ubiquitous embedded device is as unimportant to the PC as the wristwatch. I still don't think a wrist computer (or an iPad) will be so dominant people say, "remember when we used to use keyboards?"
Everyone may have a "device," but they will still use their laptop every day.