Apple’s iPhone OS 4.0 announcement delivered plenty of features we expected, and some we didn’t. In fact, it adds 100 new features in total, and they’ll all be in users’ pockets this summer. Developers get 1500(!) new APIs that provide access to parts of the phone they’ve never been able to use before.

Multitasking:

Jobs explained that multitasking took so long because Apple had to do it in a way that wouldn’t crush the iPhone’s performance and battery life. He says they’ve accomplished that goal in iPhone 4.0, and foreground apps shouldn’t be significantly slower when background apps are running.

Jobs brought out the developers of the Pandora music app, which used the new multitasking APIs to play music in the background (just like the iPod app does now). The developers of Skype also turned up to show how you can now leave Skype running in the background to accept incoming calls. Impressive. Multitasking also includes “local notifications,” which are like Push, but don’t need to use Apple’s servers.

Folders:

Finally, you can organize your apps into folders, instead of spreading them out across a half-dozen pages of home screen. It works the same way editing your home screen works now: press down on an icon until it wiggles, and then drag-and-drop. Jobs also demoed the ability to change the wallpaper on your home screen. Nice!

Better Mail:

Mail.app gets a big boost in iPhone 4.0. The rumored unified inbox, just like in the desktop version of Mail, is now a reality. The new version of Mail also adds the option to organize email by thread, support for opening attachments in the appropriate app, multiple Exchange accounts, and more.

iBooks:

You knew this one was coming. It’s the iPad’s reader/bookstore, adapted for the small screen.

Enterprise features:

The iPhone will get a bit more business-capable in version 4.0. The aforementioned multiple Exchange accounts are a good step, but better data protection and VPN support have been added, to make the iPhone secure enough for work.

Game Center:

Social gaming on the iPhone? Aw yeah! Jobs bragged about the iPhone’s 50,000 games (more than the PSP and the Nintendo DS, although more doesn’t necessarily mean better), and the then introduced Game Center. It’s a gaming social network that brings multiplayer gaming, with leaderboards and achievements, to the iPhone. Think of it as a kind of iPhone Live Arcade.

iAd:

The average user isn’t going to care about iAd, but developers should pay attention. Apple’s new mobile advertising network is a response to the ads developers have been including in their apps to make money. Steve Jobs flat-out said “We think this kind of advertising sucks.” iAd is the alternative. Its ads will be interactive and won’t take you out of the app you’re using. Jobs demoed a Toy Story ad that included games, videos, posters and showtimes (using the maps API!) for the movie. These ads are basically well-designed apps-within-an-app. He then went on to show a Nike ad that lets you design and order custom shoes, right on your phone. We are a LONG way from “Shock the Monkey” banners here, folks.