I'd be pooping my pants if I was an PC manufacturer besides Apple.
Regarding "is the tablet a PC?". I understand that for a LOT of people, they use their computer for work. To make things, write things, and do complicated stuff; spreadsheet management, graphics, research etc etc. Many of them do their work on their own computer, and many do it on a work computer and have a second computer at home.
Some of them can get by on just their computer at work. Even more people do not do ANY computer work. There are a lot of people who don't need a full computer, because their full sized personal computer, (what is traditionally defined as a PC), is used for the following: news, facebook, music, pictures, games, personal email. That's it. And an ipad does all of that, and does it well.
I know firefighters. They use just an ipad and an iphone for their computers. My friend is an engineer. He does all his work AT work on a workstation. He does none at home.His laptop broke; he is ipad only now; personal stuff that needs a big screen happens at work. My mom and my aunt are stay at home moms. They do not use their husbands desktops at home, they use their ipads. And their husbands do all their work at work, but feel no need to upgrade now since their desktops work. But my dad just bought an ipad...and now the desktop is only for home video editing.
The ipad is not going to replace the traditional PC completely. But it is going to fulfill the needs people thought they needed a full PC for. For 1/2 the cost of a macbook, and the same as decent Windows laptop/PC off the shelf, the ipad offers a lot of the same functionality with better battery life, built in program, video, and music acquisition, printing and email and video and cloud photo and automatic phone syncing etc etc...and it is all wireless, easy to use/learn and essentially virus free.
Every stay at home mom, some kids with a computer lab at school, every grandma, grandpa, janitor, cop, firefighter, secretary, gardener, waiter, waitress, cashier etc etc...does not require a full size PC for their essential online daily life.
Many, including myself, got tired of constantly upgrading, managing stuff, and optomizing. I wanted to zen my computer life. Less function, more ease. More doing, less checking/fixing. I went from PC to mac (2002). I went from a MBP to a work-only imac and MBA (2010). Now I do all my work on a MBA with some on my friends computer (2011). next time I upgrade, I will be able to do ALL of my work on my MBA. This was unthinkable to me as late as 2 years ago.
A full home desktop rig is really needed only for games, and intensive or complicated work at home. Of course its also needed for school and term papers but Apple clearly has long term plans for that.
What percent of the PC market is the above? I really don't know. But I think we are going to find out. Computers have passed the 80/20 rule...you can now get 80% of you digital life done with 20% of the effort/cost. To get that last 20% is going to cost another 80% of effort/cost. And Apple is the one making it possible. The traditional PC market is possibly about to shrink to mostly professionals and offices.
That is why I'd be pooping my pants if I was an PC manufacturer besides Apple.