A computer to me is a device that allows you to compute extensive calculations and increase work efficiency for tasks.
But it does not do what a computer is properly intended to do and to me it will never replace a proper computer.
You make some good points, Tommy.
However, not everyone is an engineer. Most heavy-duty engineering software does not run on netbooks (and nobody says a netbook is not a PC). In fact, it can be argued that most consumer PCs are not fast enough to run top-end engineering software.
Also, desktop PCs are unable to do many things an iPad can do, including going along on meetings for note taking, web browsing in bed, running on battery for long periods of time, being useful for taking videos and photos, GPS and navigation, baby-sitter, portable bird guide with songs, tea timer in the kitchen - the list goes on. So, the definition of what a PC is and "what it can do" (or cannot) could be different for different people.
The fact that the iPad is replacing netbooks (a "PC") indicates that an increasing number of people is considering an iPad as a kind of PC. Based on sales numbers, I think we have to change our definition of what a PC is.