My iBook turns off when plugged into the AC adapter?

The scary thing is that I've already replaced the DC-in board already. My model is the iBook 900Mhz 12". I reset the PRAM and NVRAM, and PMU as per the instructions on Apples' support page. The timeline of trying to get this iBook working again was after I replaced the hard drive. When installing 10.2, the battery was getting low, so I plugged the AC adapter in, which caused it to shut off. Since then I've replaced the battery, the DC-In board, and the AC adapter with no success.

I guess you tried the AC adapter without the battery installed with the same result ? Last troubleshooting step would be to disconnect all the cables on the logic board but the video cable and trip the two pins in the power switch socket to boot the machine. If you still have the problem then the logic board is bad. I often test logic boards with nothing connected only an external monitor, heaphone (to hear the chime) and a known working dc in board. If the board boot to the question mark screen then most of the time the board is OK and something else is causing the problem.

Yes, with no battery and just the AC plugged in, it's the same result. I guess I might dissemble it once more, leaving only the display plugged in. You mention "tripping" the two pins (I know which ones they are though) that are responsible for the power button. How would I go about doing this?

simply disconnect the power switch cable connector (blue & white wires) from his socket then use a small jeweller flat screwdriver and briefly short the two pins inside the connector, the iBook should start. If everything is disconnected from the logic board I use headphone or external sound system to listen to the boot chime.