Because of this thread, I immediately decided I should just insert the batteries only when using the keyboard, not giving them he chance to corrode. I don't use it that often. I was horrified to find that already I could not get the third battery out, and immediately made an appointment for the genius bar, about an hour from my home.
Imagine how stupid I felt when the man pointed out that the batteries were already out. There are only two, not three. The contact inside looks just like the head of a battery! I wasn't sure he was correct until he showed me the tiny picture of two batteries close to the cap, right in the crease where the tube meets the keyboard.
I'm thankful I didn't do any more drastic action than to bang the tube pretty hard against something, and hope I don't see any problems in the future from my abuse.