do you have the charger plugged in? the phone will turn on when the charger is plugged in while its in an off state.
mine does, every time. without fail. if i want it off i have to unplug it.
on 2.2 it used to just show a battery charging icon and not boot into android, but now it boots every time.
i didnt know if it was just my phone or not but i mean thats whats causing it for me, so look into it.