Here’s a link to the keynote presentation.

Here’s a short summary:

Seven Tentpole Features of iPhone OS 4.0:

1) Multitasking – the ability to run more than one app, have them continue running in the background, and switch between them with tremendous fluidity.

  • Background audio (example: control Pandora and stream music while in other apps)
  • Voice over IP (Skype) including the ability to receive calls when the phone is locked.
  • Background Location – optional periodic GPS tracking.
  • Push notifications
  • Local notifications
  • Task completion after exiting an app, for instance to finish a photo upload.
  • Fast app switching that keeps apps from draining battery but makes it possible to switch rapidly between apps.

2) Folders- have up to 2160 apps instead of just 180! And can dock the folders so from the video it looks possible to have 36 apps on the dock.

3) Mail - enhanced Unified Inbox for multiple email accounts and message threads.

4) iBooks on iPhone

5) Enterprise business features like better data protection.

6) Game Center – Social Gaming Network.  Invite friends, Matchmaking, Leaderboards, and Achievements.

7) iAds - Mobile advertising that keeps the user in the app but provides an emotional and interactive experience.