I'd guess that Apple's plans for the iBook are a couple of standard issue G4 jumps. They've got 1.25, 1.33, 1.42, and 1.5 GHz G4s to pick from. I'm thinking 1.33s followed by 1.5s. I'll go with the October/November guess, but wouldn't be suprised if it didn't happen until spring.
I also think that these updates hinge on the accuracy of the dual core G4s rumored for the PowerBooks. There is a nice bit of parallelism here. Prosumer desktop: dual processor PowerMac G5s. Consumer desktop: single processor iMac G5s. Prosumer notebook: dual core PowerBook G4. Consumer notebook: single processor G4. works out nicely, eh?
When/If the dual cores hit the PowerBooks next year (creating a nice 'marketing gap' between the prosumer and consumer lines), the iBook can have the screaming fastest G4 available (hasn't Moto/Freescale projected a 2 GHz evolution?), but untill then, expect very modest revisions in the ibook line. Apple can't have iBooks and PowerBooks too close, spec-wise.
At least that's what the voices in my head have been saying... among other things...