Moore's law is now commonly understood as referring to the best transistor density we can do. But in the original paper Moore was talking about the density of the least cost process. With the original meaning Moore's law stops not necessarily when the density stops increasing, it can also stop because the new finer process remains forever more expensive than the previous one. With the old definition some expect Moore's law to stop at 28nm. Some even say it already stopped at 40nm (which is still less expensive than 28nm, so we'll have to see). The difference matters, because of economics. New fabs are always more expensive, but up to now the resulting chips were both better and cheaper. So everybody moved to newer process eventually, and the addressable market also increased. If the original Moore's law stop due to increasing chip cost, you can still get better perfs, but now it's more expensive. As a result, all the applications where performance is good enough and price matters more (embedded applications, ...) will stay on the now cheaper bigger processes. So the new fabs will still be more expensive but will address a shrinking high perf market. This will have consequences.

Was it actually about density? I always thought it was about transistors per dollar. Sure, density is a very effective way of lowering costs, but the maturation of technology at a specific process size will work just fine too.