When using the device for prolonged periods of time, such as when you're talking on the phone, charging the battery or browsing the web, the device may become warm. In most cases, this condition is normal and therefore should not be interpreted as a problem with the device.
The device has a protection feature that if the device becomes too warm (due to use of GPS or Bluetooth while charging, using the device as a wireless modem, etc.), the auto protection feature will be triggered and the charging process will be stopped. Once the device cools down, it will then begin charging again.