From what i can read of this and from own experiance the "voltage" referred to is actually dropped to change the clock frequency not the actual working voltage of the cpu, having said this i would need to see the data sheet for the ARM device used in the N900 to be able to fully understand why someone said overclocking is "lowering" the voltage.
Voltage is not the reason you get heat on any electronic component it is actually the current being drawn that creates heat.
Please understand ohms law for this.