Over the weekend I read several reports from various sources claiming that the iPhone 3GS was getting abnormally hot - hot enough to discolour the case, apparently.
I'm not saying that these reports are wrong, or somehow invalid, but I've been using my iPhone 3GS constantly and consistently for the past 16 days and haven't noticed anything like this.
Perhaps I'm not doing something that others are doing, but last Friday I drove for an hour with my 3GS in a Belkin TuneBase. The iPhone 3GS was charging, playing MP3s, using GPS and the magnetometer, linked to both 3G and EDGE networks depending on availability, and making the occasional phone call via a BlueAnt Bluetooth speakerphone. I arrived at my destination, pulled the iPhone out of the dock and did not notice any heat issues whatsoever.
I've read that many are claiming to have hot iPhones after extended periods of Wi-Fi or 3G use in combination with heavier apps (like games). So I played FireMint's Real Racing for 20 minutes, linked to Wi-Fi with Notifications enabled and e-mail push active. The phone did get somewhat warm, but no more so than I would have expected from simply handling the device for 20 straight minutes.
Just prior to these experiences, I wound up stuck on a sweltering plane for several hours while it sat on the tarmac at BWI. I played several games on the 3GS, wrote some emails, read some news, and surfed the Web (including sending a friendly note to the airline), all over 3G. The plane was incredibly hot, but the iPhone wasn't. I also used my brand-new Mophie Juicepack Air for the first time on that plane, which provided a welcome power addition and got me through to the other end of my flight with plenty of battery life to spare. It was at that point that I got in my car and drove home, as mentioned above.
All in all, from my arrival at the airport to my arrival at home, my iPhone 3GS was in use for the majority of six hours, with pauses for takeoff and landing. It was even charging some of that time thanks to the Juicepack Air.
Well, the plural of anecdote is not data, but the above anecdotes represent my experience. Perhaps there are a bad batch of iPhones out there that have overheating problems, maybe those that have wrapped their 3GS phones in rubber cases are enhancing the overheating. I've read that some people first noticed the problem when they kept the phone under their pillow (why you would do this kinda escapes me, but OK), and lack of any airflow exacerbated an issue. I really don't know, but my experience is unlike theirs - so far, anyway.
But then again, I may be an anomaly. For instance, I've read several reviews and comments on Apple's MagSafe adaptor fraying, melting, and sparking. I've read about Apple MacBook Pro batteries dying after a few months, and battery health declining to the point of uselessness after 80 cycles. I read these things and note that I've never had anything of the sort happen to me across several MacBook Pros, MacBook Airs, all the way back to a 1GHz G4 iBook that still gets 2.5 hours of battery life from the original, five-year-old battery.
Maybe I'm blessed. I certainly can't say that I treat my laptops and cell phones with kid gloves, because I certainly don't. I've had batteries go dead on me in the past, I've had laptop hinges break, including my first MacBook Air that developed a case of wiggly hinges after a year - Apple Care fixed that for free, however. I'm no stranger to odd, annoying problems with new technology - but I'm also sure that I haven't seen any overheating problems with the iPhone 3GS. If I do (and I'm going to try), I'll definitely let you know. Also note that I generally turn Push and Notifications off to conserve battery life. I don't really need to know the second that Michael Jackson's parents apply for custody.
p.s. The Wired article on the overheating and case discolouration has a great comment from Mizar: "Over time, a faint yellow image of Steve Jobs appears, a la the Shroud of Turin."
UPDATE: I just spent the last 30 minutes running my iPhone 3GS hard and measuring the surface temperature with an infrared thermometer. Here are the results:
Ambient temperature: 75F
Sleep, not charging: 88F front/back
1m recording video: 88F front/back
1.5m recording video: Front 89F, back 94F
6m recording video: Front 89F, back 94F
Following that, I ran:
4m with GPS/magnetometer/Wi-Fi while charging: 94F front, 98F rear
15m with GPS/magnetometer/Wi-Fi while charging: 96F front, 98F rear
Then I went back to the home screen and locked the phone, testing it after 5 minutes:
5 minutes after sleep, charging: 92F front/back
8 minutes after sleep, charging: 88F front/back
The temperature does increase with heavy use, but it barely reaches body temperature, even at the end of all the heavy testing. In fact, I kept placing the phone to my head as if I was taking a call while running these tests and may have inadvertently increased the temperature on the front by simply holding it against my ear.
None of these temperatures are anywhere near what would be required to discolour a case or make the phone too hot to use. Then again, I wasn't running these tests for several hours in the Sahara under a pillow in a toaster oven. Yes, the phone gets warm with heavy use, but certainly not hot.