http://www.youtube.com/watch?v=zvTRKKS0wpo <--- NOTE: Ignore the desc. "In Iowa a family says in April their son's Apple iBook made a popping noise, filled the room with smoke, and bursted into flames." NOTE: This is caused by defective Li-ion batteries! NOTE 2: That was not my iBook, and I do not live Iowa.
This is caused by Li-Ion batteries. This applies to every laptop model with a PowerPC CPU.
What to do if your laptop has a defective battery: Take out battery in laptop and use laptop on Power Adapter only. OR Take your battery to BatteriesPlus and exchange the batteries inside for Ni-MH batteries to put in. (If it's really possible.)