Occasionally I have exactly the same problem,
- freshly fully charged battery will not power n900 on.


This is Exasperating when you know for a fact
that the battery has been charged completely.

After a few encounters of this sort
I eventually decided the -detected- battery voltage floats too high,
up to where circuitry
kills the turn on. (pure guesswork actually)
Temperature does seem to have a substantial effect on this as well,
charging in a cold room seems to set it off.

In the past few months I shy away from leaving it on charge
and simply avoid fully discharged and fully charged states.
I charge it during breakfast,
and turn it off completely if I use too much battery.

This is not a solution or a cure,
it is simply procrastination until I get a real replacement.
I am guessing the cure involves dis-assembly and
cleaning dust from between the tracks of some forgotten cap
or resistor, or voltage sensing circuit,
if not replacing some SMD component now no longer available.

All in all,
this symptom reminds me to keep looking for an n900 replacement.

good luck