Apple has begun to rollout iPhone update 3.1.3 via iTunes today and the release fixes minor bugs involving the battery life indicator on the 3GS model, app launch issues, and a Japanese keyboard input bug that would cause certain apps to crash out.
To update your device:
1) Launch Apple iTunes
2) Connect your device to your computer with the USB cable
3) Click to the device’s page that appears inside iTunes (it may open for you automatically)
4) Click the button Check for Updates
5) Follow the on screen instructions which will walk you through updating the device.
Be sure to allow iTunes to fully back up the device for updating, in case there are any issues with the new update.