I don't press the power button (assuming the device is already active). If the battery becomes low when I'm using the N800 I just plug in the power cable and the only visible screen difference is that the battery in the status bar begins to "fill up" as the device is charged. There may also be a brief on-screen message in the top right corner confirming the device is now being charged.

Unfortunately I don't know what to suggest as the problem you describe is quite unusual and I've never seen it reported before. Have you tried reflashing the device just in case a software setting has somehow become corrupt?