I think the term gets thrown around a little too loosely. Nowadays, if CPU X get 120 fps and CPU Y gets 115, CPU Y is lauded as a horrible bottleneck and a worthless piece of crap worthy only of being a paperweight.
To me, a bottleneck is something that will choke off a significant amount of performance. Something like a 2.2 GHz dual core running a pair of GTX580's would be a bottleneck as youd be losing probably a third of your potential frames per second.
I think its all relative to what youre using. If youre running an Athlon II X3 on a single 6870, then yeah a 3570K would give you a few more frames but it wouldnt be enough that youd notice in actual game play. However, those same two procs powering a pair of GTX670's and youre talking a pretty big difference and that would be a true bottleneck to me.