Hi, and welcome to Apple Discussions.
Things I have recommended in the past:
I think I'd try a sweetly scented fabric softener sheet (the kind you use in the dryer) closed in the iBook at night and see if that will at least alleviate the stench somewhat.
If my keyboard were one of the smelly ones, I would try using
Renuzit Super Odor Neutralizer on it in this manner:
Remove the keyboard. You don't have to remove it completely, just lift it up and flip it over and lay it on the palm rests and trackpad. I wouldn't put any liquid over on the left side where the shield over the innards is perforated, but on the right underneath side of the keyboard, I would try dipping a Q-Tip into the Renuzit Super Odor Neutralizer and swabbing the under side of the keyboard. Allow it to dry over night before reassembling.
If this doesn't do it, I would repeat the procedure, and, in addition, dab the center of each key with the Odor Neutralizer. Again, allow to dry over night.
You may have to repeat the procedure if the smell comes back.
I have not had this problem, but this is what I would do (if I did) before resorting to buying a new keyboard.