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