The problem here is that you are misusing the word 'overclocked'. An overclocked processor means that someone is running a processor at a speed higher than what it is officially rated for. For example, running a processor rated at 1ghz @ 1.2ghz.
However, you first need to understand how processors are manufactured and then rated. Believe it or not, every single G4 chip that Motorola sells comes from the exact same silicon wafers (barring different CPU revisions). One a processor is stamped out of that wafer, it is rigorously tested to determine is quality. Even though 20 CPU's might come from the same wafer, that doesn't mean they are all of the same quality. Some might only safely reach 1ghz. Some might go all the way up to 1.6ghz, or even higher. Some might not go past 500mhz, and have to be scrapped. Once a 'safe' range is decided for a processor, it is given a rating. All that rating means is that it is guaranteed to run at that rated speed.
Of course, just because its rated for one speed, doesn't mean it can't go higher. If a company has extremely good CPU yields, they might end up sticking higher rated CPU's into slower machinges and "underclocking" them. That is basically how I was able to push a 800mhz G4 to 1.4GHz with no issues and no excess heat generated.
In response to your question about the G4's in the Powerbook and iBook - yes, they did come from the same silicon wafers. One is rated at 1.5ghz while another is rated for 1.33ghz. They are identically the same exact chip, but one is just able to run at a higher speed more reliably. There is no 'overclocking' taking place here. Apple is simply following the speed ratings given to them by Motorola/IBM.
I hope that clears things up.
