For tablets to catch up, they need to do more than just get more powerful and solve the text entry problem. The capabilities of a larger system that draws 20 or 30 watts are always going to be several steps ahead of a thinner, more portable system that requires only a few watts to operate. As time goes on and tablets get more powerful and capable, so too will laptops and desktops. Tablets will rise to meet our current computing needs, but our computing needs will evolve, and some of them will only be met by larger, more powerful PCs.
Even Apple seems to acknowledge that the "post-PC world" will still rely heavily on a personal computer. Sure, the company could stop requiring that initial iTunes setup, but consider the new AirPlay features of iOS 4.3 Apple knows that all those videos, photos, and albums you have simply won't fit on an iPad: you'll keep that stuff on your PC and stream it throughout your home.
The PC Isn't Dying, It's Just Being Redefined
Just what is a "personal computer", anyway? Is it defined by the operating system it uses - Windows, MacOS, or Linux? Does a physical keyboard make it a PC? A processor that uses more than some arbitrary amount of power? Is it size? It can't be size - a MacBook Air is a personal computer, and it's about the same size as an iPad. You could easily argue that today's smartphones are personal computers, as they're easily as powerful as the PCs of five or six years ago.
The world isn't moving away from the PC, it's just transitioning from the PC being defined as a "personal computer" to "pervasive computing." Computers will fill our lives with specialized capabilities in various form factors, sizes, and locations. Our lives will be seamlessly infused with computers that are tailored to, and fulfill the maximum capabilities of, their processing and storage capabilities. In our pockets we'll have computers that can make phone calls and run the kinds of applications that work well on small screens, and it will have all the capabilities you can get from the tiny power draw required for that device. That will scale up all the way up through computers that sit on our under our desks, to the huge servers that power the cloud services and applications we all use, drawing so much power they give the local power company a headache.