Apple just announced iPhone OS 4.0 today.

1. The main features are
2. Multitasking
3. Folders
4. Improved Mail
5. iBooks for iPhone and iPod touch
6. New Enterprise Features
7. Game Center
8. iAd

and 100 new user features including

File & delete Mail search results
Search SMS/MMS messages
Web search suggestions
Choose image size in Mail messages
Bluetooth keyboards
CalDav invitations
Larger fonts for Mail, SMS & alerts
Cell data only setting
Spell check
Persistent Wi-Fi
Gift Apps
Birthday calendar
Recent Web searches
Create Playlists
Top Hit in search
5x digital zoom
Nested Playlists
CardDav
Tap to focus video
Upload workouts to Nike+
Places in Photos
iPod Out
Home screen wallpaper
Search SMS/MMS messages
Wake on wireless

Available summer, more info here

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