I managed to get my n900 into a reboot loop last week after installing the tweak app (forgot what its called) that allows you to change the grid snap for lining up desktop icons among other things. Anyway, right after I installed it, I began re-arranging the shortcuts/widgets on my desktop. I was trying to place the Media Player widget right beneath the status bar, and somehow the widget jumped ON TOP of the status bar and forced the device to shutdown/reboot. Whenever the desktop started to reload after booting up, the Media Player widget would try to load at its last known location, right on top of the status bar....reboot...reboot...reboot. I just followed the instructions at http://wiki.maemo.org/Updating_the_firmware
to reflash and now all is well.