It's not that hard to convince Apple to replace machines outright. If you've replaced key components 3 times, they seem to be willing to do it if you just complain for a while. G3 iBooks are prevalent for this sort of occurrence. One of the employees at work is getting his Rev A 15" AluBook replaced with a shiny new 1.5GHz because the LCD died within a week of purchase, and the logic board (or parts of it) have failed twice. It seems that the machine still functioned, but ports would cease to work. He even put in some extra money to do upgrades on it.
As for trading it for the Cube, not a bad deal. G3 laptop for a G4 desktop. Don't listen to those naysayers who insist that clockspeed is more important than vector processing, especially in OS X. I have a Cube, and though she's getting a new logic board (it was problematic from the start, hence why I got it so cheap), she's a gorgeous machine nonetheless. As for the ethical issue of giving it to the store, and washing your hands of it, you really ought to tell them that the logic board is going to go, that it will be covered under the replacement plan, and how many times it has failed in the past. Knowing that, they may be able to get a G4 iBook out of the deal.
