Nothing that would require a complete update to be released. Is not Apple moving too fast towards OS 4.0? With iPhone OS 3.2 released days before iPad launch and now the firmware updated to 3.1.3 with a minor fixes and improvement like the battery display issue fixed for iPhone 3Gs.  

Here is what is displayed as the “updates” in 3.1.3 when you install it:

• 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

Fixing a few glitches and releasing it as an update is something beyond my understanding. Why create so much buzz around it?

Nevertheless, for those who have unlocked and/or jailbroken their iPhones or iPod touch SHOULD NOT upgrade to firmware 3.1.3 because no jailbreaking or unlocking tool has support for 3.1.3 yet. If you want to get locked again and wait till dev-team launches support for 3.1.3 then sure go ahead and upgrade.

Download iPhone Firmware 3.1.3

Jailbreak iPod touch 3.1.2 with Sn0wbreeze

Download Sn0wbreeze for Windows