Yes, actually someone has. The 500 and 600mhz G3 iBooks were easily modified thanks to very, very simply re-soldering of wires.

After that all other G3's and G4 iBooks were very complex to overclock and really no one tried it. That was until the 1.2 and 1.3GHz iBook G4's came out. They use the same chip as the 1GHz eMac. If any of you know the 1GHz eMac chip (Short lived and now only used in iBooks) had sixteen "relay switches" around it's perimeter. By moving the switches into different positions the CPU becomes faster, or slower. If I recall correctly the top speed anyone got out of a 1.33GHz iBook was 1.75GHz and it heated up very bad.

I also recall a Japanese site modding a 14'' iBook G4 to 1.70GHz and adding a very large amount of thermal gel to the CPU and piggybacking off the battery connector a small fan that was burried beneath the thermal gel that was hooked to a cheap big mouth straw that sucked air onto the CPU from the Airport Compartment.