Here are the changes from version 3.0.1
Source : Apple - iPhone - New features in the iPhone 3.1 Software Update.
and for those with jailbroken and unlocked iphones here's what the dev team has to say about it ...
Improved syncing for music, movies, TV shows, podcasts, and photos
iTunes U content organization
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
Source : Apple - iPhone - New features in the iPhone 3.1 Software Update.
and for those with jailbroken and unlocked iphones here's what the dev team has to say about it ...
If you update to Apple’s new software using the normal iTunes process, you will lose your ultrasn0w unlock. In fact you may lose it permanently, because for most people the baseband firmware cannot be reverted to a previous version (unlike the main application CPU firmware).
But don’t worry…our PwnageTool program lets you update your main firmware without touching your baseband firmware, so you can still have the best of both worlds. But you must be diligent about saying “no†to your iTunes request this week to update your firmware.