Post-PC era doesn't mean you won't need PCs. It's about different types of devices, that you use differently and interact with differently and that don't try to be the same thing as PCs.
For many, it could eventually replace the PC.
At one point some businesses had large refrigerator sized computers with vacuum tubes like my old employer and they finally brought in a Windows 3.1 computer with Pentium to replace that old set of computers with tubes. Back then, I thought computer technology was moving fast but I could have never predicted something like iPod/iPhone/iPad.
The only thing I can't do with iPad 2 is type that fast. And if Apple puts OS X on it, then some can call it their lone machine and have it fill all their needs. I have modest needs and the Mac mini I have does all I need and then some. I am sure there's somebody with only an iPad and quite happy with it.