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, 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. 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.