Ok let me make this clear.

Reflashing your N900 will NOT void warranty - especially not if you use nokia tools and nokia firmware.

It would be ridiculous from Nokia, providing you with the tools, the firmware, and a handset build on opensource technology, and then telling you you are not allowed to reflash or mess around with it.

This is not your regular mobilephone, the default rules do not apply here.
This is Linux - you don't get f*cked for hacking/messing/reflashing it - that's what you are supposed to do, and it's encouraged my nokia.