The price differentiation aspect (which fianchetto describe pretty well) has recently shifted somewhat from clock speed to number of cores. Except for a few very cheap processors Intel has their entire lineup in the 2.8 to 3.6 GHz range, it's approximately the same for AMD.
There is no doubt that AMD is currently pushing their top 4 and 6 core models to the limit of what they can reasonably sell as stable.
What Intel is doing with their Sandy Bridge CPUs is more noteworthy, the current top model of that line is a 4 core clocked at 3.4 GHz, but overclocking results are in the 4.4 to 4.8 GHz range with stock cooler at stock voltage, for a top model that is an extremely high overclock. This suggests that they could release a 4 GHz model with very little effort.
However, if you ask Intel marketing Sandy Bridge is not their top architecture, as that spot is taken by the older 6 core Gulftown. The problem is that Sandy Bridge is just so much better that it's hard to justify the 2 extra cores more than making up for the difference, but according to marketing strategy they have to. So in order to keep that claim somewhat credible they handicap Sandy Bridge artificially.
If Intel weren't as far ahead of AMD as they are they would push their hardware harder, but since AMD doesn't have anything to match the Sandy Bridges as they are Intel let the quirks of marketing decide what to sell.