I'm thinking the display on the X is a HUGE power hog.
This is precisely what I'm thinking. Between 100% brightness, GPS constantly pinging satellites, navigation rendering the 3D-ish map, and 3G downloading all the map data, its pulling more than the standard 500mA that USB provides. I have also seen it disable the charging due to it getting too hot, but that was actually reflected in the status bar icon. With Navigation running and the screen at 100%, it said it was charging, but slowly lost power.