In spite of their name, desktop PCs often have several users. Laptops, netbooks, and tablets are usually single-user machines—that is, they really are personal. Modern mobile operating systems are built with room enough for one—Apple's iOS and Google's Android are both tied in to a single user's e-mail, calendar, and app-purchasing accounts. Forrester's numbers also suggest that in the future we'll have many such machines around the house. Your "main" computer will be a laptop—and you'll probably have several smaller, tablet-type machines that you use regularly as well.
My laptop is my "main" computer, my only computer, at least until my current wireless carrier contract ends and I'm able to land a Google Android. Then I'll have two single-user machines, four and five if the PS3 and Xbox 360 qualify, six if I dare include my Moleskine journal.