Apple released an update for the iPhone. Release 3.1.3 fixes some issues like;

• improves accuracy of reported battery level on iPhone 3GS
• resolves issue where 3rd party apps would not launch in some instances
• fixes bug that may cause an app to crash when using the Japanese Kana keyboard
• Tethering is back (even on the non exclusive carriers)....
  Results may vary since not every carrier allows tethering (like T-Mobile in the Netherlands).