neither Apple nor AT&T mention better service quality! on the jb side both Dev team and Geohot have said it's not worth the time or effort if your already 3.1.2
this is All Apple is reporting the update does!
* Improved accuracy of reported battery level on the iPhone 3GS
* Resolves issue where third-party apps would not launch in some cases
* Fixes a bug which may cause an app to crash when using the Japanese Kana keyboard.