iPhone OS 3.1 also includes these features and updates:

* Improved syncing for music, movies, TV shows, podcasts, and photos1
* iTunes U content organization1
* Redeem iTunes Gift Cards, codes, and certificates in the App Store
* Display available iTunes account credits in the App Store and iTunes Store
* Save video from Mail and MMS into Camera Roll
* Option to "Save as new clip" when trimming a video on iPhone 3GS
* Better iPhone 3G Wi-Fi performance when Bluetooth is turned on

* Remotely lock iPhone with a passcode via MobileMe
* Use Voice Control on iPhone 3GS with Bluetooth headsets
* Paste phone numbers into the keypad
* Option to use Home button to turn on accessibility features on iPhone 3GS
* Warn when visiting fraudulent websites in Safari (anti-phishing)
* Improved Exchange calendar syncing and invitation handling
* Fixes issue that cause some app icons to display incorrectly

http://www.apple.com/iphone/softwareupdate/