Although the thread is quite old, I want to post have some answers to the problem described initially.
What's happened to the n900?
In my case it was subjected to overvoltage due to broken car charger (13V).
Why doesn't it boot in normal mode?
BSI sense circuit is fried. So battery detection fails while booting. I don't know which component is the problem (may be bsi opamp?), but the bsi pin seems to be tied to 0.6v and adc value is in range 241-245

In my case bq27000, charging circuit, usb are still working. 2.5v supply and adc seems to be ok. Low battery alerts can be disabled by hiding .desktop file from battery applet. Sometimes I need to restart bme to initiate charging. So the device can be used, but it may not be safe since I don't know what else is blown.