I first read about this feature of the 750FX in this article at xlr8yourmac.com. Since then Apple has made it more difficult to change the iBook's speed. It seems that current versions of OS X continually monitor the CPU speed and set it back to normal if it is changed.
iCook gets around this by patching the kernel image in memory to disable its ability to set the CPU speed. It then sets up a sysctl called hw.cpuspeed that allows the CPU speed to be set and monitored by the user. iCook makes no permanent changes and everything goes back to normal after a reboot.
I am able to run my 700 MHz iBook at speeds from 300-800 MHz, although 800 MHz is not entirely stable. CPU performance scales nicely along this range as can be seen in this graph of results from Apple's Skidmarks GT benchmark. iCook allows speed settings from 300-1200 MHz in 50 MHz increments and comes with the full GPL'd source code.
For more information see the ReadMe file.