Multitasking: It's here, finally. It's handled with a simple task switcher: double click your home button, and you get a list of running apps. Select, switch, done. Multitasking is limited to audio streaming, VoIP and GPS apps, as well as a few other allowances: they can finish specific, important tasks in the background, for example. As far as non-music/nav/VoIP apps, those can be suspended in the background, but not left running. (See below.) Full details here. Fast app switching: With iPhone 4's multitasking, most apps aren't actually running in the backgroundjust certain functions of the app, like an audio stream or a GPS lock. But! All apps can now be frozen, in full, so that when you reopen them, they're restored to exactly the state they were in when they were closed. Local notifications: Notifications can be sent between apps on the phone, not just from remote servers. In other words, if something important happens in an app you've opened and moved away from, a notification will pop up in whatever app you're using at the time, effectively saying "switch back to me!" It's a fairly clever way to keep track of multiple apps without the need for a start bar or dock-type interface. From Apple's dev guidelines: The advantage of local notifications is that they are independent of your application. Once a notification is scheduled, the system manages the delivery of it. Your application does not even have to be running when the notification is delivered. Apple's official line: iPhone OS 4's new multitasking offers users a new way to quickly move between apps, and provides developers seven new multitasking services to easily add multitasking features to their apps. These services include background audio, so apps like Pandora can play music in the background, and VoIP, so VoIP apps can receive a VoIP call even when the iPhone is asleep or the user is running other apps. iPhone OS 4 provides multitasking to third party apps while preserving battery life and foreground app performance, which has until now proved elusive on mobile devices. And some more technical details, again from Apple's developer guidelines: An application can request a finite amount of time to complete some important task. An application can declare itself as supporting specific services that require regular background execution time. An application can use local notifications to generate user alerts at designated times, whether or not the application is running. App folders: Now you can sort your apps into folders! That's homescreen clutter solved, just like that. Apple's description: Folders help users better organize and quickly access their apps. Simply drag one app icon onto another, and a new folder is automatically created. The folder is automatically given a name based on the App Store category of that app, such as "Games," which the user can easily rename. Using folders, users can now organize and access over 2,000 apps on their iPhone. 2160, to be exact. A new Mail app: Unified inboxes, multiple Exchange accounts, fast inbox switching, threaded messages: These new features are actually a huge deal, since the iPhone's mail client has barely changed since 2007, and Apple doesn't allow alternative mail apps. Apple's pitch: iPhone OS 4 delivers the best mail experience on a mobile phone with its new Unified Inbox, allowing users to see messages from all their email accounts displayed together in a single inbox. With just a few taps, users can quickly switch between inboxes to see messages from any single account. iBooks: Oh hey, that iBooks ebook reader app and accompanying ebook store we first met on the iPad has ambled on down to the iPhone. Nice, since you can now take your books with you wherever you go, as oppose to wherever you go with your iPad. Custom backgrounds: Jailbreakers have them. Hell, the iPad has them. Now you can choose a persistent background for your iPhoneand not just for the lockscreen. Game Center: Apple's going to roll out a centralized gaming servicea multiplayer network like PSN or Xbox Liveto help connect games to one another, by the end on the year. There are 3rd-party services that already do this, like OpenFeint. They will probably die. Full details here. iAd advertising: It looks like Apple's finally making use of Quattro, that mobile ad company it gobbled up a few months ago, by rolling out its own advertising platform, a turnkey ad plugin for app developers called iAd. The theory here is that instead of relying on links to external websites, which pull users out of apps whenever they tap on an ad, developers can use Apple's new tools to keep people in the app while still showing them advertisingsort of like popover browser windows. You can watch videos, play games, and even buy apps from within these ads. This is in the iPhone OS 4 developer tools, but it's not explicitly a part of OS 4, so you won't see apps with iAds until later this year. Full details here. 5x digital zoom: Could this hint at a higher quality camera in the next hardware? 3.2 megapixels seems a bit low for 5x digital zoom. Bluetooth keyboards: Another carryover from the iPad, Bluetooth keyboard support will finally come to iPhone 4. A bevy of other new developer features, including 1500 new APIs to play with: See here for more details. The Hidden Features Now that we've had a few days to use the OS, we've compiled a huge list of features not covered in the keynote. Which Devices Get It, and When? When the software ships in the summer, iPhone 3GSes and iPod Touch 3rd-gens devices will get all of the new features. The iPhone 3G and Touch 2G will get "many things," which doesn't include multitasking. I repeat: the iPhone 3G won't get multitasking, ever. iPhone 2Gs will be left on a hillside somewhere to die of exposure, or something. Apple didn't drop any clues about the next iPhone's hardware, but it's a fair bet that we'll see some changes come June.