I think this distinction is well worth drawing. If Apple buy a G4 chip from Mot rated at 1GHz, then Mot are saying they warrant this will work correctly at that clock rate within their defined temperature range (typically up to 70 C). If Apple then push that chip to 1.25 or 1.5GHz and it fails (either totally or erratically), Mot will not care in the slightest because they never claimed it could do it. This would leave Apple with "overclocked" chips performing unreliably and with no comeback from their supplier, but with an obligation to their customer to make it work. Apple would then gain a tremendous badly reputation - "fast and crash" or something like that. Apple simply wouldn't do this.
If Apple ship a chip running at 1.25GHz, then it's because Mot have supplied it on the basis it will run reliably at that clock rate within its temperature range. Anything else is commercial suicide.
[But to contradict myself slightly: if Apple used one of those wonderful "fridge coolers", and hence could be certain the chip never got abov, say 30C, Mot might well warrant a higher clock rate - the back of chip data sheets sometimes has a nice frequency vs temp chart showing what is warranted and what is not. But given the new DPPM only has heat sinks and fans, there isn't (IMHO) the certainty of keeping the temp down enough to know they will work.]