Now I know that the temperatures inside my car don't get high enough to even begin to melt the plastic of the bottle... So where is that flavor coming from? Well, I did a bit of web reading about some polymers today during lunch (what, you don't read about polymers for fun?) and came up with this theory:
The degradation of the bottle's PET (polyethylene terephthalate) is leeching out trace amounts of acetaldehyde into the water. This is that bottle taste. Any [armchair or pro] chemists out there who can chime in on this one?