Steven touched on a good point,

For how a tablet can be used, the "end" point of hardware is pretty close. Desktop sales have declined a lot mostly because, IMO, there is no real need to upgrade for the average person. Workstations and gamers need to iterate their hardware to keep up with software demands, but for surfing the web, using office, and watching movies, a Conroe is all you need. So someone that bought a computer 4 years ago will never need to upgrade. Tablets are getting to that point really quickly.

Eventually we may get to a point where the only reason to buy a new PC is if the PC physically breaks. PCs are good physically for 10+ years under normal use, probably even longer. I wonder how the industry will adapt once people figure out they no longer need to upgrade to do what they do.

Increasing iPad functionality will eventually eat into Mac sales. Apple can't make the iPad powerful enough to do things a normal computer will do, but they have to iterate to make people keep buying the newer model. I'm interested to see how they go about doing that.