On April 8th, Apple finally showed the most exciting version of the iPhone's operating system—OS 4.0.

OS 4.0 adds 1,500 new developer APIs and 100 new user features to the existing iPhone OS platform, while during the announcement, Jobs mainly focused on what he referred to as seven "tentpole" apps.

1. Multitasking
Background audio, VoIP, Push notifications, Local notification, Task completion, Fast app switching.

2. Folder App
Drag Drop UI, Intelligent naming, now you can have 2160 apps on iPhone

3. Enhanced Email
Unified inbox, Multiple exchange, Threaded messages & Fast switching

4. iBooks Support
EBook Reader, iBookstore, Buy once read anywhere, Sync page & bookmarks

5. Enterprise
Data protection, Mobile Device Management, Wireless app distribution, Multiple exchange accounts, Exchange server 201, SSL VPN support.

6. Game Center
Social gaming networking, Invite friends, Matchmaking, Leaderboards, Achievements

7. iAd – Mobile Advertising
Interactive ads, Ads within app, ads built into iPhone OS, Apple sell and host ads, 60% revenue to developers

The software will be available this summer. It will be fully supported on the iPhone 3GS and third-generation iPod touch models, while the second-generation iPod touch and iPhone 3G will support "many" but not all of the new features. Prior-generation devices, including the original iPhone and iPod touch, will apparently not support iPhone OS 4.0 at all. The initial release of OS 4.0 will be for the iPhone and iPod touch platforms only, and iPad users will receive the new OS in the fall.