As always, Engadget provided excellent coverage of the live press event in their live blog: http://www.engadget.com/2010/04/08/live-from-apples-iphone-os-4-event/?sort=oldest

Touting over 100 new user interface features, here is a brief overview of what was covered:

• MULTITASKING!
  
  
  • Apple approached multitasking by adding a task dock that displays when double clicking the home button, kind of an ALT-TAB for the iPhone
  • Apps can be paused or can continue to run in the background. Apps like Pandora can continue to play music and can be controlled via the lock screen
  • VOIP Apps like Skype are now fully integrated and act much like the regular Phone app
    
  • Navigation apps can continue to run in the background and provide voice directions while other apps are running (including music)
    
• FOLDERS!
  
  
  
  • Applications can be combined into folders to reduce paging through applications
  • Can be auto named by the category of the apps or custom named
  • Can be put on the dock
  • Folder icons shows mini icons of the apps they contain
    
• Unified Mail
  • Mail from multiple accounts can be viewed together
  • Can be organized by threat
  • More than one Exchange account
  • Open attachments with apps
    
• Ability to change the home screen and lock screen wall papers
  
• Create playlists
• 5x Digital zoom
• Tap to focus for video
• Gifting apps
• Geotagging
• Places in the Photo Application
• Bluetooth Keyboards
• Spellchecker
• New privacy settings for applications using location
• iBooks
  • iBooks for iPad is now being made available on the iPhone
• New Enterprise Features
  • Data protection
  • Multiple Exchange Accounts
  • Exchange 2010
  • SSL VPN Support
• Game Center
  • Basically XBOX Live for the iPhone – invite friends, matchmaking, letterboards, achievments
• New Mobile Advertising (iAd)
  • Good for developers, probably will annoy users

Though all of these are great advances for the iPhone OS, the following things are still on my wish list:

• New Home Screen showing a summary of phone, email, etc activity
• Ability to change Safari search engine to Bing
• Other browsers, I.e. Opera and FireFox

(Images from Engadget)