I experienced exactly the same thing. Lot of random reboots in the first hours of using it. Then the five dots, first animated and frozen afterwards. I made the biggest mistake in leaving it in that state and let the batteries run empty so it was impossible for me to do a firmware flash. I considered going out for a external charger for the battery and do the firmware flash again but the constant reboots had made me worried too much. In result I sent the device back to Nokia after having it in house for only a few hours

I still think the n900 is a killer device but I'll check back in a few months when the child diseases has been sorted out.