Maybe this has to do with powering on when the machine is connected to the charger? When the N800 (770 too) is turned off and then connected to the charger, it powers itself on in a special mode (presumably to monitor the charging). When it boots into this mode, or gets out of this mode (because you just took the charger out), it is indeed impossible to power it on.

Apart from that, my N800 has always started when I press the power on button long enough. I have to wait after the blue led below the direction keypad is lit.