Same Problem Here
I am running a 400/G3 DV Graphite and I just experienced a similar problem. I pushed the power button nothing happened. After pushing it several more times, and still nothing. I noticed a slight flicker in the corner of the modem but nothing else.
I unplugged everything, installed a new battery on the logic board and still nothing.
Then about 6 hours later on a whim I pressed it and the green light actually came on, I pressed it and held it and it came on. I backed up everything I could (it shut down again for about 20 min.).
Then started diagnosing it with Disk First Aid and Norton Disk Doctor. A bunch of files were fixed but most of them were date related and my computer did revert to 1969 (I wasn't even alive yet).
It is still running four hours later but I'm not sure why. In my case it seems like a power supply problem, maybe inside the computer so I am going to have it looked at tomorrow. So I will keep you posted. But everything I've read so far says check the periphrials first, then try zapping the PRAM, then replace the battery and if all that fails, take it in for service.
Cake or Death?
I'll have cake please.