Heck, for my work I'm often using hundreds of cores on a cluster. Or I'm using 1 core and need hundreds of GB of RAM feeding it. Its all over the place. However, I was under the general impression that the i7 2600 benefits quite greatly from improved architecture that makes it more efficient even though its a similar clock speed with 2 fewer cores.
Mostly I'm thinking about what I read here:
http://www.tomshardware.com/reviews/core-i7-990x-extreme-edition-gulftown,2874-1.html
So, for most tasks it just doesn't seem to make much difference if you have the 990X or the 2600. Then the 990X is $1000, vs. the 2600 at $300. That means by the time you buy it from apple you're probably looking at $3000 iMac.
Now if you know you sit in a spot where you can use all 6 cores pretty much all the time, that might be the right choice for you. However, even if you do have well paralleled tasks the 2600 is not far enough behind to much too much difference. And if your work is moderately mixed between things that can use 6 cores, only 2-4, or even just one, then there's a good chance the 2600 comes out ahead.
Anyway, get what works for you, but it seems pretty obvious why the 990X is not in an iMac. But things may change, maybe Ivy Bridge will bring 6 cores to the iMacs?