I disagree. There will always be desktop pcs. They are as cheap as hell and i can't think of anything that is cheaper. As long as they are the cheapest form of pc they will exist.
Netbooks are cheaper, and it won't be long before tablets are too.
I think once those devices are capable of replacing the desktop for the average user, which isn't that far off, the market will shift away from the desktop form factor completely. Bathroom internet is the killer app of the future, after all. There's a big opportunity there for iOS and Android to take users away from Windows.
Notebooks will have a much longer life, and Windows will be on nine out of ten of those machines, but the typical big beige box has one foot in the grave already.
There will always be desktop workstations and monster gaming rigs, but the consumer desktop will get displaced by other devices sooner rather than later.