Well, I installed the shim and put it all back together again, and the iBook is humming along alright. A few lock ups that are indicative of the logic board problem, but on the whole, it runs mostly stable. I reloaded OS X 10.5 and iLife and it was stable enough to do that plus update. It was a much easier job given that (a) I’d done it before with my 800 Mhz iBook G4 and (2) I wasn’t too vested in completing the job; if it crashed and burned never to work again, well, I wasn’t really out anything. Much less stressful…
So, we’ll see how it runs long term. Christmas came early for a fried of ours; I’ll be bartering the lappy for some baby-sitting time. w00t!