Former Google CEO, Eric Schmidt in a recent interview with CNN answered this question pretty well. When asked "So, does that mean the PC is dead?", he responded:
Well, the PC will be used for things where you need to do a lot of typing. Because it's hard to type on that glass screen. But, for everything else, you're going to be using a mobile device - a tablet or a mobile phone or some kind of mobile, small thing that you'll carry around. They're inherently better. They're more personal. With your permission, they know who you are, they can make suggestions to you - that kind of thing.
So, the PC, which, remember, grew out of the IBM desktop model sitting in a corporation, makes perfect sense for somebody who sits at a desk all day. A lot of people don't.
A few years back I was excited about the new laptops being released. As they have gotten faster, each new generation seems to offer less new than the rest. We had longer battery life, cheap graphics that could play HD content and affordable light chassis to keep things interesting the past few years. Sandy Bridge and Ivy Bridge really don't interest me all that much. I do not spend my time running benchmarks.
But compared to mobile devices, like smartphones and tablets, they are getting boring. What the software in our laptops do has not really change all that much since good old Windows 98. Mobile devices have made really interesting uses of their camera's, from barcode readers to augmented reality. GPS have made these devices really smart, allowing you to keep informed of the local weather and conditions around you. "With your permission, they know who you are, they can make suggestions to you..."
The laptop will always have a place in my set of tools. A 13-inch screen is a size for carry and viewing. I do type a lot, so I will need the physical keyboard. But I think I will keep my devices longer and longer. I do not see myself replacing my late 2011 13-inch MacBook Air until it physically dies on me. The main advantage of my MacBook Air to my other laptop, a HP Compaq 6510b, was 3x the battery life, better graphics, SSD drives and half the weight.
It is going to take more than current Sandy Bridge MacBook Air or next years Ivy Bridge version to convince me to upgrade. Maybe drop another half a pound and give me another four hours of battery life and we will talk, well maybe.
On the other hand, I do not know how long I will hold on to my HTC Desire HD, when the new dual core Android 4.0 (Ice Cream) handsets with their giant 720p screens come out. Now that gets me excited. I am sure the iPad 3 will cause a stir with the tablet crowd too. My next laptop... am guessing I am not buying another one till 2013.