RealWorldTech published a fairly technical article this week that discusses all the aspects of 3D integration and its implications for processor performance. ARS Technica just published a more summarized version of this article.
Here's a snip:
Since the beginning of the integrated circuit, chips have been 2D affairs. A network of transistors and wires is etched on a square sliver of silicon, and as fabrication technology improves, those transistors and wires get ever smaller. But as transistors get smaller and more of them are crammed into the same-sized square of silicon, the relative distance between them grows larger. To see what I mean, consider two 1µm (micrometer) transistors joined by 1mm of wiring. A signal that travels from one transistor to the other will cross a distance equivalent to 1,000 transistor lengths..