Is humidity a factor? Are Xbox 360s in Florida just as likely as a 360 in Seattle?
Humidity is a co-factor with temperature for many failure modes. The hotter the room ambient conditions, the more likely a 360 is to fail, all else being equal. Same for humidity.
No shit sherlock. Thanks for letting us know that hot, humid air is less efficient for cooling.

Is keeping the 360 horizontal more safe than keeping it vertical?
I don't think so. Vertical exposes more surface area and volume to heat exchange with cooler room air. And I think opens more vent holes. Just don't let it fall over.
Yes, setting it vertical would expose more surface area to ambient air, but the hardware's volume has nothing to do with it (and obviously remains constant). He's also negating the cooling affect that the table can add as a heat sink. You
think that it exposes more vent holes?
Think?!? Just look at the f'n thing, genius!
Not exactly an insightful article. Was it a secret that 360s aren't exactly well built and have insufficient cooling? Maybe MS will learn from their mistakes and their third console will be well built and competently engineered (like Sony).