I hate to bring up this term in a thread full of nerds (me included, not being a jerk!), but a lot of it will have to do with marketing. People have been convinced they need to buy P4's for email and web surfing, but the jury's out on whether or not Intel can convince mom and pop that they need a dual core for the same task. I leave AMD out because they do little marketing to consumers.
Now, onto the "technical" portion. If Intel's dual core offering was the only thing out there, seeing how badly it performs and all, AMD may have never come to the table with their design in the consumer arena and we'd be looking at single core desktops for a long time to come (leaving dual cores in the server lines). The intel dual cores don't seem to be selling all that well even with the huge marketing budget. If AMD's design can find its way into more OEM's, or Intel copies/borrows from it, there's a good chance dual core will be the only thing out there in desktops in 3-5 years.
Not to mention, clock speeds have leveled off. Dual cores are probably cheaper to produce than the R&D in new materials or architecture to improve performance. That's probably where the marketing dollars will go, therefore.
Most people buying PC's arent us. They're folks who buy what other people tell them they need, because they know little about the technology. It's really more of whether or not they can be convinced.