I have a similar issue (but I got lucky, because the battery was not fully discharged so it was capable of booting up the device despite of low battery signals): when I connect the charger to N900 in it's powered off state, the LED blinks orange, and the Nokia logo shows up followed by the "knight rider lights" on the screen but at this point the boot process intervented and the phone shuts off itself. After a few seconds it tries to boot again, but this time the usb symbol is on the screen. "Knight rider" comes again, and the phone shuts off finally. Same issue with manual (by the power button) powering up.

Charging from a PC / laptop works fine.

But there is an interesting part: if I take off the battery for a second and put it back and powering up the phone without a charger connected, it powers on flawlessly. And when the OS is running I connect the charger it charges correctly (I hear the whistling noise from the usb adaptor, which means to me "it's charging correctly on high current".).

I cleaned the usb port contacts by pressed air and a tootbrush, but no success.