Tablets will have to improve a lot before they can replace desktop computers.I think people who talk about a post-PC world underestimate the needs of the typical consumer. They're not all teenage girls whose only computing need is Facebook.Think of all the students who need to write essays, the business people who need to use spreadsheets, etc. I know plenty of people who can barely use a computer, but whose needs would preclude using a tablet as their sole computing device.
I don't think we are living in a post-PC world because PC's are still used for many important things that you just can't do on a tablet. I think this time is coming and tablet sales are increasing year after year and PC and laptop sales are decreasing. People still feel more secure on physical computers over tablets but times are changing quickly.
The reason why I believe we are not living in a post-PC world is because of how connected we are to our machines. I think, more than ever, people spend the majority of their leisure time on the internet on things like Facebook or YouTube as opposed to spending time with their friends or family doing outside activities. While some might consider the fact that we rely heavily on machines like tablets, phones, or laptops as evidence of not being tied to a physical PC anymore, I believe that this is just an extension of how indocrinated we really are in terms of engraining electronic devices into our every day life.
Right now, PCs are still needed to do the "heavyweight" tasks. People can surf the web, chat, and watch videos using devices like smartphones and tablets, but a PC is still needed to "get down to business." I don't believe this will always hold, and the PC will eventually become a niche market, but that point isn't here yet.
We are not living in a post-PC world, I believe we are living in a PC world right now. I think there is movement away from PC's and closer to mobile and alternative devices but I think the majority of our time on computing devices is spent on PCs. They are still a very important part of our society and the industry is growing. The more technology we discover, the more we depend on our PCs.