Those are basic CPU parameters in Linux, and we can adjust them (among others) through kernel-power-settings in N900's with Kernel Power.
Google them, if you curios more about them.
Sample rate speaks for itself, up treshold regulates that when is neccessary to set the next higher CPU clock.
I have it in 25, so 25% of CPU load is enough for it to set itself from 500 to 1000MHz.
I haven't noticed any affects on battery life, so I'm using it only with this two freqs allowed.