I don't think you are likely to see the desktop ever go away, really. Sure, phones and tablets will become more popular...but technology won't sit still, and as the tablets and phones gain abilities, so will the desktop computer.
A good portion of this talk about the PC dying is actually a marketing campaign designed to sell you tablets. I wouldn't spend too much time worrying about it--tablets just don't do everything well that desktops do.
Tablets are great for surfing the 'Net, for instance--but not so great at all if you want to write, and writing is still one of the most common activities undertaken on the computer, everything from letters and emails to stories to novels to technical reports...all of which a tablet or a handheld are just poorly designed to do.
Also, these portable solutions are designed more and more to depend on data storage in the "Cloud"...and storing data in the Cloud isn't all it's cracked up to be. There are legal and privacy concerns which just don't apply if your data is on a hard drive actually sitting in your house. Nobody is going to blink, for instance, at using the Cloud to keep their list of website favorites...however, you *should* blink if you are thinking of saving your tax returns in the Cloud, or your banking information, or that report on confidential patient data from work, or any one of a thousand other things that could be life- or career-altering if they fell into the wrong hands.
The desktop is here for the long haul. Don't write 'em off just yet.