Battery won't charge/ Black X on battery icon?

If you haven't already, I'd Google "Coconut Battery", download it, and run it. It will give you some detailed information that might let you determine whether it's the battery or the machine. The power adapter may be showing green because the computer can't charge a bad battery, so if it shows a max MAH capacity of 0, for example, then it's a bad battery. I know your old battery is dead, but when you put it in your machine, does it behave differently, or does it show the same "X"? If it behaves exactly the same, that's evidence that it's the computer (and that your board may be bad, given all you've already replaced), and if it behaves at all differently, that's evidence your new battery is bad.

97% of battery issues, especially with iBooks, turn out to simply be a bad battery. The problem is, most people don't have enough batteries around to do thorough testing. I have found an iBook here and there that was not able to charge a battery due to a problem with the board, but it's pretty rare.