No worries, down to 2,5V is absolutely safe, and N900 "lose consciousness" at ~2,8V, due to not enough power for anything.

Just be sure to not leave battery in such state for long unused, as it will easily discharge itself below that level, and even to absolute 0 (which, in turn, is very lethal for LiIon). At below 3V charge inside is very low, and it takes really quick to self-discharge.

/Estel