I have the same problem w/ a 300Mhz iBook. It doesn't boot any more, black screen, no chime, HD and CD spin, but no data is read. PRAM and Power Manager reset won't help. The iBook definitely has no backup battery, but afaik a capacitor instead that might be broken. I wonder if the ROM could be broken? I don't think it could be the internal RAM, because in case of faulty RAM the machine beeps one, two or three times.
Could you fix your iBook? If so, what did you do? If not, what do you plan to do?