June 30, 2009, 9:10 PM — According to a report on the Web, a newly-released iPhone 3GS became so hot during normal use, that its white case turned a bit brown. While this toasted iPhone has generated a lot of Internet buzz, let's keep it in perspective. So far as I know, it is the only instance of this happening. In other words, with Apple having reported sales of 1 million iPhone 3GS models during its first weekend of sales, this overheated iPhone is one in a million. As such, I wouldn't worry that this is going to happen to your iPhone 3GS any time soon--if ever.
More common (at least as reported in a variety of reader forums) are reports of iPhones running hot--but not hot enough to do any damage. The main complaint is that the iPhone becomes sufficiently warm so that actions, such as putting the phone to your cheek to make a phone call, become untenable. PC World'sMelissa Perenson and David Coursey have both described personal experiences with a too-hot iPhone 3GS.
If your new iPhone 3GS has ever had this symptom, you might want to take a deep breath before assuming that either you need a replacement iPhone or that Apple needs to issue a recall.
First of all, this symptom is not new to the iPhone 3GS. It has been an occasional issue with all previous iPhone models. For example, a Cnet article from April describes a heat problem with iPhone 3G models running iPhone OS 2.2.1. According to this report, the heat could reach a point where Wi-Fi connections were lost. Still, many users have never had any such symptom. Even those phones that have heated up appear to run well after cooling down and likely do not require any hardware fix.
According to Apple, if the internal temperature of your iPhone gets to a point where the "operating temperature has become too hot," a message stating the "iPhone needs to cool down" should appear on your screen. If you have never seen this message, your iPhone is likely fine--even if it sometimes feels warm to the touch. If you have seen this message, your iPhone is probably still okay. Yes, I assume it's possible that the iPhone could overheat to dangerous levels without generating this message, but this is not common.
Still, there does appear to be a non-trivial issue here. As again reported by David Coursey, and as I can personally attest, Apple Geniuses can use diagnostic software on a Mac to check for serious iPhone overheating. Apparently, if the program shows that an internal heat sensor has been tripped, you may qualify for a new phone.
What seems to be causing the less serious, but still annoying, instances of hot iPhones? While some users attribute it to an "overheating" battery, I believe it is more likely an overworked processor. In my own experience, I have had this symptom on several occasions, even while my iPhone was asleep. When I went to grab it, the phone felt unusually warm. In addition, the battery charge had declined, unexpectedly fast, to the point of needing a recharge. This only happened if I had left some app active when the iPhone went to sleep (not all apps caused this, but certain ones did). Just as the iPod app can continue playing music even after the iPhone has auto-locked, apps can continue to put a drain on the processor during this state.
Similarly, Macworld's Dan Frakes finds that his iPhone can heat up when doing processor-intensive activities for an extended time, such as playing "high-end" games. The phone gets warmer than usual at these times, but not dangerously hot. Another user found that a problem with Outlook SharePoint alerts led to his iPhone running warm.
If there was a way to check on the real-time status of iPhone processes, such as Activity Monitor can do for a Mac, this would be relatively easy to diagnose. Alas, Apple has not provided us with such a tool. So we have to do the best we can.
My best advice is to always return your iPhone to the Home screen if you expect not to be using it for any extended period of time. This almost guarantees that no runaway process can overheat your phone. If you currently have a too-hot iPhone, returning to the Home screen and letting it sit for awhile should allow the phone to cool off. If not, powering off the phone until it cools down should certainly do the trick. While some users have put their iPhone in a freezer, to accelerate cooling off, I don't recommend this.
Otherwise, if you have never or almost never had any problem with a too-hot iPhone, know that you are far from alone. Happily, most iPhone users appear to fall into this category.