Thank you Brian for your explanations. I see what you mean about all the energy having to be dissipated (the Io) under no load. Only a small percentage of the watts in is used to actually turn the rotor, so that other large portion of energy can't just go poof; law of conservation of energy says it's gotta do something or go somewhere, so it turns into heat.

I'm leaning towards explanation number 1, just because it seems to me that it would affect the motor's temperature far more than speed (especially if the motor is mounted inboard like on a 4wd touring car, and air flow is limited). Airflow is a huge factor in cooling though, I can't doubt that (though air is supposed to be a pretty crappy fluid for heat transfer I suppose).