wow..
You know, that would be quite the feat if you managed to do it. i know i have seen g4 logic boards on ebay for dirt cheap. you really might be able to do this.. the only problem i could forsee would be the way the logic board is mounted in the case. i know the cases appear to be exactly the same, but is that similarity only skin deep? try it and let us know! (btw, what size screen would this iBook be?)