This may be a dumb question, but, was the different battery that you tried pulled out fully charged up and working, from a different phone, or was it a spare that sat in a drawer since you bought the N900? Your battery is my prime suspect, I've had a spare BL-5J die through being left discharged and it behaves in a similar fashion... in my case, it pretends to charge (green light), but never actually develops enough juice to boot the phone - reproducible with the knackered battery on 2 handsets. I don't know the state of play with after-market repairs in TX, but I suspect that this will either be something you fix yourself, or something that can't readily be fixed.

Second dumb question: you are trying to charge it from the proper wall charger, not from USB? And being patient enough (5-10 minutes) for the battery to recover and start charging normally if it is going to? I've known it take a minute or two for a phone to register enough juice to report that it is charging... If none of that helps - and it really will not boot with a fully charged battery that is known to work - then you do indeed have a problem I don't think I've seen before.