iMacs have 7455 CPUs

On theirs new iMacs technical documentation (ici) it appears 1 & 1.25 GHz G4s have a 256 Ko cache. This is the proof they are 7455 and not 7457 as they'll have 512.
To minimize dissipated heat, Apple probably chose to use 1GHz 7455A and the brand new 1.25 7455B. Overclockers will be delighted, as those two CPUs are easliy overclockable.
Also gives us a clue as why the iMac line has changed while the laptops didn't.
So that they don't lose on battery lifetime or CPU heat, the laptops need 7457 CPUs. These problems are easily solved on desktop machines.