Coding Horror has a great article on Choosing Dual or Quad Core. Nicely done because it's simple, basically a list of screenshots of the CPU meter playing various games and doing various apps, and letting you know how much improvement you get if you go from a dual core to a quad core (something I've been thinking about).
Hint: not worth it unless all you do is run either rendering software (POV-Ray) or benchmark apps. The list of apps isn't huge either, at least, it didn't include my Battlefield 2142 crack. Interesting to see that about half the the apps tested actually lost performance from 2-4 cores.
It all comes down to the applications. I have a quad core system that is vital to our office.... its running Virtual Server and 3 dedicated virtual machines. I see benefits during high peaks of usage where the CPU switching would seriously slow down performance. Hence why I went quad core.
I can see how its not critical for desktops though. Unless you need the rendering power... chances are you aren't maxing out the CPU as it is.
I sort of expected some of the more modern, hardware punishing apps (ie: games) to take more advantage of multi-cores. IE: one core does AI, one does physics, one does rendering, etc. I guess though what small parts get broken off to what core need to be decided at coding time, and you don't design for 16 cores when the standard is 2 or 4. Either that or the OS should be better about distributing load across the system.
I hope this trend will change (at least by the time I finally get a quad core machine :)