I had a similar problem, bright orange LED flash on a dead N900 followed by weak red flashes when plugged in.
I wiggled the battery contacts with a safety pin and reinserted the battery, started with about 60 seconds of steady orange LED then the normal pulsing orange.
<few minutes later>
I can now boot up, must have done something when frequently switching between a prepaid sim and the sim from my laptop modem.
I will still need to replace the fall damaged microUSB port to make charging reliable, it requires being set on a book with the cable drooping a bit to charge.