brhmac: Overclocking is the practice of forcing a microprocessor to run faster than its factory-rated speed, usually by manipulating the bus multiplier and/or clock frequency. This is possible in some cases because chip manufacturers regularly factory-rate chips at speeds lower than what the chip could actually handle.
When Motorola announced the current line of G4s (7447 and 7457), they stated that they would be factory-rated up to 1.33GHz. However, in the first production runs, they had problems with getting many chips that were usable beyond 1.25GHz. They were also having problems with yields of the 7457 line, which differs from the 7447 only in that the 7447 can't support an external L3 cache. This is supposedly the reason why the Powerbooks dropped the L3 cache when they moved to the newer chips; rather than going with the 7457 in all Powerbooks, Apple has reportedly been using a mix of 7447s and 7457s.
The fact that PowerLogix is now selling several 1.42GHz 7457-based upgrades hopefully indicates that Moto is getting a handle on these production problems and is producing G4s in quantity at higher speeds. This would also mean that Apple may have access to higher-speed G4s now.
What may make a speed bump just a bit better than a 10% clock boost would be a move to 7457s across the board, with a reintroduction of a L3 cache. The 512K on-chip cache mitigated the effect of losing the L3 cache, but the G4 has always been starved for memory bandwidth, and I'm sure that adding the L3 cache back in would boost performance.
Note that in the past, Motorola has supplied Apple with G4s that ran significantly faster than the chip was originally designed to run. Hopefully that's not what they're doing here (shipping overclocked 1.25GHz 7457s), since in the past the extra factory-rated speed came at the cost of significantly higher voltages and heat dissipation. OWC (and Giga, and other upgrade vendors) are selling 1.3 and 1.4GHz G4 upgrades based on this older, hotter 7455 chip -- which, when announced, was never planned to ramp far beyond 1GHz.
In any case, I agree with brhmac -- a 10%, or even 20% (with L3 cache) speed bump isn't nearly enough to make the Powerbooks compelling. Hopefully Apple and IBM are on track to getting 65nm (or underclocked 90nm) G5's into the Powerbooks soon.