To my knowledge the new iBook's all use the same cpu and motherboard. The only difference between the three models would be the cpu being set to a different multiplier (size also for us 12 " users). If this is the case, overclocking the 2 lower model iBooks should be safe to or near 1ghz.
Also considering that the newer 2002 iBook's can be overclocked through software, this might mean that the new iBooks could be overclocked through software as well. This would be great for all the warranty lovers out there. Technically there would be no way for Apple finding out you overclocked the cpu because you could easily set it back to default values.
My question now to everyone is... Has anyone tried overclocking it yet? If so, how? And if what I've stated were to be true, would you overclock your iBook?