Just a small nitpick. While the state of the art in x86 land might be 3GHz, the IBM Power chips and I think the Z mainframes too, not sure on that, have gone far beyond that speed.
The limit to how fast you can clock a CPU is mostly due to how long the chains of CMOS devices run before they hit a cocked latch. Having one CMOS drive 10 other devices takes longer than having one CMOS drive only 1, so there's a standard metric called the FO4 (Fan Out of 4) that can be used to measure gate delay. IBM has often put more effort into having low fo4s for its processors (usually around 15 or) vs. x86 (around 20).
POWER6 chips reached 5.0GHz in 2008: http://en.wikipedia.org/wiki/POWER6
POWER7 chips however have been clocked down to 4.25GHz: http://en.wikipedia.org/wiki/POWER7