Interesting question - one that it would be nice to get a really definitive, scientifically-based answer to.
A couple of weeks ago my wife and I drove down to Virginia to visit relatives. Along the way, we stopped for a few hours to take the tour of Monticello (which I highly recommend). I had my 4x5 with me along with quite a bit of film, and I knew that the kind folks who run the tourist concession at Monticello were not about to let me in with either the tripod or the big pack - so it had to stay in the car. I cracked the windows, and found a parking spot that was quasi-shade, but even so the car was warm when we finished the tour. Fortunately, I cannot see any difference in the fog level in my film.
Several years ago, I did a workshop with a well-known large format photographer who regularly left all of his film in his van. He wasn't at all concerned about heat and fog.
On the other hand, I had an experience similar to Ara where the UPS guy left the shipment (from B&H - Adorama isn't the only retailer who use UPS) by the side door - on the blacktop driveway in the afternoon sun in June (hmm - three strikes?). In that instance, the film (T-max 400 bulk roll) had a distinct blue fog after processing. I know that coincidence does not make correlation - but I can't think of any other explanation for the fog.