If my Sprint HTC Hero is idle the battery temperature almost always reads the same temperature as the room that it is in which is usually around 25 degrees C to 26 degrees C (about 78 degrees F) in the warmer months and around 19 degrees C to 20 degrees C (around 68 degrees F) in the colder months. If I am using my handset heavily by say, multitasking a lot, and using a program that persistently runs such as my IRC app, it will usually hover between 38 degrees C and 40 degrees C (which is about 102 degrees F) in the warmer months, and a few degrees cooler in the colder months.
The highest I have ever seen my phone was about 44 degrees C (about 111 degrees F), but this was when I was outside on a very hot day and I was using it heavily again as stated above. Once the heavy usage stops, the temperature reduces fairly quickly.