Multi core is going to be a case of diminishing returns faster than the mhz bumps were under today's circumstances. There's only so much you can thread out before the cost exceeds the benefits. At some point, the process will be "fast enough" and adding in another core won't be worth the expense. Even games, especially games, have a point where there's more cores than threads. If you could thread out core gameplay, physics, graphics, sound, and AI, you're talking 6 processors total counting one for the OS. Considering Graphics and Sound already have dedicated chips, 6 would allow you to do processor intensive stuff in the background as well.
The only way Multi-core is going to need to move beyond 6-8 cores is if we move to an age where there's a single computer running screens with inputs throughout the house. If a single computer is supplying an entire family's worth of needs, then we could see a need for more than 6-8 cores, but still that would likely top out around 10-20 cores.
While that is a very possible, even probable, sceneario it's still unlikely any user will need any more than 6-8 cores at any given time.
I'm guessing Quad-Core will be where it stops though.