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