1)Apple sells this phone in northerly climates (Canada for one)
2)Apple specs that it can be (when turned off) in environments down to -20 Celsius
3)I don't think anyone will argue with me that the nominal purpose of a cell phone, is as a communication device that a person CAN CARRY AROUND WITH THEM.
Combining these 3 facts, I think a reasonable person would conclude that they can take the phone in and out of the house with them when it is warmer than -20 C outside.
Thus, It seems reasonable that the warranty should still apply when this "reasonable person" has taken the phone in and out of the house at, oh, say -15 C
However, this test shows that doing so can trigger the humidity sensor, thereby voiding the warranty. Even though the person has not done anything unreasonable.
The think that I think some of you (who live in warmer climates?) are missing is this: the environment changes used in this test simulate normal daily use for those of us who live in colder climates.
Also, I doubt this issue is limited to iPhones: I had at least one motorola phone's warranty voided by the water sensor, even though I was unaware of having ever gotten the phone wet. This article could finally explain that issue as well...