OK as no one is giving you solid answers here i will butt in and try to help.
Have you gone to the Wiki page all about flashing the N900? here is the link in case you need

http://wiki.maemo.org/Updating_the_tablet_firmware

Try a cold flash and do everything exactly as it tells you to do and if after that lot you have not succeeded to get back to running order then i am afraid to say you will have no choice but to return it to Nokia care.

Good luck i hope it works for you without having to return to Nokia.