A modern Intel/Athlon CPU should be close to linear at the top end of its performance range, but even for those chips, as you scale back the clock, you'll see that you were just on the flat end of a logarithmic curve. You're right, there isn't a rule of thumb that can be applied, especially since power consumption can vary not only by the chip and its stepping version, but even by the program(s) that are being run at the time.
Regarding the problems with leakage current, its means that Moore's Law is a dying bird. Sure we'll get around the problem, and get silicon down to 0.06 or even 0.03 microns....but its a lot harder and slower than people originally thought. The leakage problem is forcing a redesign on the actual shape of transistors (they're not being made just smaller, but with a different geometry on the N-P boundaries). Also, the smaller circuits are having immense problems with cross-talk and signal integrity, making not only the chips in need of redesign, but the software tools used to develop the chips as well.
Finally, we're also seeing some issues with the materials themselves. For instance, for 20 years, we've been used to the concept of "infant mortality" on chips. If they died, they died young...otherwise they would run forever. But copper is replacing aluminum as an interconnect within the chip itself-- a technology that was heralded as the 'savior' of Moore's Law, to allow us to reduce resistance and voltage quickly. Well it does, but copper has turned out to have the annoying problem of having migrating voids in the interconnects, a problem that may turn a good chip bad only after a few thousand hours of use.
All these problems will be solved. But we're not seeing the easy gains we once did. Look at Intel itself-- a year ago exactly they introduced a 2ghz P4. By Moore's Law, a 4ghz chip should be due in 6 months. Graphics processors (ala NVidia) were a couple years ago outpacing Moore's Law. But now, Nvidia is 3 months late in taping out their NV30, primarily because of yield issues on the 0.13 micron process.