It's likely you already know a lot about iOS 4 -- we've covered it in depth in the past, when it was still called iPhone OS 4. Obviously the big additions are multitasking, folders, iBooks, along with user-defined wallpapers, Mail's new unified inbox, Bluetooth keyboard support and some 1500 other features, most of which require updated apps in order to really shine. Here's a quick list of the biggies -- you can get most of this stuff on your iPhone 3G or higher / iPod touch 2G or higher right now if you're a dev, and it'll be free for everyone on June 21st:
- Background audio (think Pandora).
- Background VoIP (think Skype).
- Background location data, both with live GPS for backgrounded turn-by-turn, and cell tower-based for lower power draw.
- Orientation lock -- you can set it to always stay in portrait
- Spell check (like on the iPad).
- Bluetooth keyboard support (again, on the iPad).
- User-defined wallpaper (a jailbreak favorite).
- Tap to focus when recording video, just like with photos, and a 5x digital zoom for the camera.
- Playlist creation and nested playlists
- App folders for sorting apps. You can even put an app folder in the dock.
- Enhanced Mail. You can have a merged inbox view, switch between inboxes quickly, and sync to more than one Exchange account. There's also threaded messaging (at last!) and in-app attachment viewing.
- iBooks, just like on iPad, only smaller. You can wirelessly sync books between platforms, a la Kindle.
- Enterprise features, including remote device management and wireless app distribution.
- Local notifications. Like push notifications, but sends a notification straight from the app without needing a push notification server, perfect for an alarm, for instance.
- Fast app switching. Saves the state of an app and resumes it from where you left off, without dwelling in memory.
We've been using iOS 4 since it was announced on our iPhone 3GS, and it's very much the same iPhone experience with some extremely welcome tweaks, like being able to quickly switch between apps by double-clicking the home button, and being able to lock the phone in portrait orientation. But other than that it'll be up to the app developers to really take advantage of these 1,500 new APIs -- we haven't seen too many apps make use of the new features yet, and that's where iOS 4 will really shine.