IPhone :: 4S Touchscreen Damage From Overheating?

My iPhone 4S (iOS 5.1) was not quite two weeks old, and had experienced none of the overheating problems that are reported sporadically in various fora. Then, last night, I shot a 21-minute video. By the end the phone was fairly hot, though it did not shut down or display the green-triangle "overheated" image. I stopped the video and noticed that the left-hand quarter of the touchscreen no longer worked: for example, nothing happened when any icons in the left-hand column of any screen were touched. Display was unaffected, but nothing (cooling down, cycling power, resetting the phone, etc) restored the touchscreen function.

I took the phone to an Apple store today and they gave me a new one, saying this is a problem that they hadn't seen (which they always say, of course). But I'm still curious because my first stop was the Verizon store across the street, where I bought the phone in the first place. The Verizon folks told me that (1) they would not replace the phone, because (2) it sounded to them that I might have voided my warranty, on account of shooting a continuous 21-minute video, as (3) this overheating during video recording and trashing the screen was a known issue in iPhone 4Ss, which I should have known about, and at any rate (they stated emphatically), iPhones were not designed for long video shoots. Not only had I not heard any of this, I had heard the contrary many times, so I'm still wondering whether they were just trying to BS me or whether there really have been similar reports that just hadn't got publicized yet.

Info:
iPhone 4S, iOS 5.1