Is this to do with temperature? Does it affect performance? Obviously when the CPU is pushed it maxes out to 2Ghz (according to CoreDuo), but wouldn't things be faster if the the CPU was maxed out constantly at 2GHz?
Im pretty new to the mac worked but for the most part processors either intel or amd act the same across all platforms, meaning that they act according the the software or the instructions that have to be processed. I wouldn't expect the processor to run at full tilt unless your were using an app that required all of that power. And also being that your on a portable, like the last post said, it might have something to do with saving battery life.
Check what your Energy Settings are, Processor should be set to High.
I think that OS X looks at how much Raw Intel Core Power that you need and then calculates how fast the processor should run at to conserve battery power. Isn't it 2 processors anyways? So its like 2 x 1.5GHz (I know its a processor core and not just a processor but yeah...)