Apple have released OS 3.1.3 for for iPhone 3GS, iPhone 3G, iPhone 2G, iPod touch 1G, iPod touch 2G, and iPod touch 3G

Updated Features
Fix for iPhone 3GS battery level reporting
Fix for app crashes on launch
Fix for app crashes while using Japanese Kana Keyboard

And this time I do not paste the Download links You should Check thisNote.

Note: DO NOT update to 3.1.3 if you have a jailbroken iPhone / iPod Touch. There isn’t a jailbreak/unlock for this firmware yet