People will still want a larger screen, a keyboard and a mouse. Doing just about anything more than light web browsing, taking lecture notes or playing simple games is not going be very possible on something 7-9" big on the diagonal. No amount of processing power will change how we need to interface with that power.
The only way I can see it as a reasonable replacement is if the tablet essentially docks to a monitor/HD/keyboard/etc. But even then, you'd be able to get more power for cheaper in the PC box. So, it might suck off some of the PC market, but no matter how fast tablets get, I don't see them out right replacing PCs for a very long time.
Unless they eventually move us 100% to the cloud and sell us a subscription and a monitor.