First we have to understand why the iPhone, iPad, and so on are better suited for the avarage user than desktop systems. Why has Microsoft's strategy of propagating the PC as the "digital hub" mostly failed? For years, especially Microsoft (but also Linux and Mac evangelists) told us to use a PC or a Mac for sharing media and devices, for creating content and consuming media, for storing selfmade videos and photos. But this strategy has two weak points.
First, to be able to accomplish most of these tasks a PC must be switched on 24/7. Only very few heavy users are really using their PC as a server. It is expensive, a PC can be noisy, the hardware is not really optmized to run round the clock and because of lot of security updates there is still lot of administration required. So the idea of having a home server is not really catchy.
Second, even with a home server it is quite hard to share the media outside the home. Sure, there are tricks like DynDNS or port forwarding, but only very few users do understand these technology and are using them. Even worse, by opening the server towards the Internet, it becomes vulnerable, which means additional administration efforts.
Sharing media is becoming more and more common. People are putting their photos on Flickr and their videos on Youtube and send the URL to their friends. Try this with your media hub at home. This is possible, but how much more effort and knowledge is needed? Regarding copyrighted content - it is no longer necessary to have it stored on your home server. Music and videos can be stored on portable devices as well, since they have enough storage.
So the remaining points are content creation and consuming and sharing of devices. For device sharing, specialized boxes like FritzBox in Germany are much more suited than a full-fledged PC. A FritzBox, which you get as part of your DSL contract, offers a few LAN ports, wireless access point and a USB port for printer and USB hard disk where you can put all the media which is then accessable from all devices in the network. The web-based setup is very simple and still powerful; virtually zero administration is required and the power consumption is very low.
For content consumption there are devices like iPad with no boot time, with an excellent screen, which are transportable, consume very little power and the input possibilities are sufficient to enter a URL, write a short comment or chat. It is possible to connect these devices to your TV or HiFi set, so no PC is required here.
So now comes content creation. This is the area where PCs are to stay. Writing long articles, media production, coding, this is where PCs are strong and will stay for a while. But now there is a fact, that in social networks only 10% of the users are creating the content, the rest is consuming. Also these 10% are not creating all the time; they are also heavy consumers, therefore they will probably have two devices, one for consuming and one for creation, which they have to switch on, to wait the boot time, to start correct program and then start creating. So content creators will still buy a desktop, but it will be a tool for clearly defined tasks, for everything else there will be a consumer device.
Legacy does not mean that a device will disappear. Desktops and their operating systems are and will remain in businesses, they form the backbone of lot of companies and Microsoft will still remain one of the most valued companies and earn lot of money. There will still be lot of Windows versions to follow, but the excitement about them will not be much stronger than excitment of new version of AIX.
Windows vs. MacOSX vs. KDE vs. GNOME vs. BeOS wars are thing of the past. The future discussions and most exciting developments will happen on mobile devices. So watch out for iPhone OS vs. ChromeOS vs. MeeGo (and probably Microsoft if they get their act together with Windows Phone 7 and Slate). For Intel and AMD this development means that they should concentrate on server processors and very low power processors for the consumer devices, since this is the area with the most demand in the future.