Why did CPU clock speeds stop increasing?

Question

Possible Duplicate:
Has CPU speed already broken Moore's law?

Additional Reading:
Whats the deal with these new lower clock speed CPUs on Laptops? Can they still run apps well?
Is a higher core count or higher clock speed more beneficial to a computer's performance?
Clock Speed and other factors

In the 90s my parents bought me my first computer with a 166mhz Pentium processor. It was awesome at the time. 3 years later I bought myself a 500mhz processor which was better and cheaper for its time. Clock speeds increased dramatically in the 90s and early 2000s, but it seemed to stop when they hit 3ghz.

Memory speeds have increased and so have bus speeds, and processors generally come with multiple cores now, but the core CPU speed has generally been unchanged since about 2002. Why is this? Is there a physical/theoretical limitation that was hit?