i experienced this exact scenario and theorized that my n800 didn't have enough battery power to initiate a charge. i tried everything, including waiting after i had plugged it in, and nothing seemed to work. then, magically, the nokia screen stayed on longer and it showed the charging icon. this all happened after running a torrent app drained the battery faster than the charger could supply power and it shut down. the fact that this thing can't run on power from the ac adapter alone is a major flaw in the design. i suspect that nokia prioritized backwards compatability with its phone chargers to supplying adequate power to the n800. i'll probably buy an extra battery and external charger to guard against this happening again.