New iPhone features

Prince McLean has an article in AppleInsider about the latest on iPhone 4.0:

iPhone OS4 will introduce seven major "tentpole" feature and a long list of minor enhancements this summer, Steve Jobs has announced at Apple's release preview event.
The top seven features are multitasking, app Folders, Mail improvements, Enterprise features, a Games Center, and Apple's new iAd network:
1) Multitasking
The first major feature is multitasking.
2) Folders
The second major feature, Folders, involves dragging apps on top of each other to create a folder with those apps inside it. It names the folder according to its contents, a feature Apple is calling "intelligent naming". iPhone 4.0 supports up to 180 folders for a total of 2,160 apps: 12 apps per folder. The hierarchical organizing of apps into folders will make it easier to work with more apps at once.
3) Improved Mail
A third feature, Unified Inbox, lets users view all of their mail accounts in one inbox just like Mail.app in Mac OSX. The new version of iPhone Mail also offers fast inbox switching, an option to organize messages by thread, and the ability to open attachments with an app from the App Store. iPad debuted this ability, with Apple's iWork apps already able to open Office or iWork attachments within Mail.
4) iBooks for iPhone and iPod touch
A fourth feature brings iPad's iBooks to all iPhone OS devices. The iBook application will work on both iPhone and iPod touch, and all books and page placements will be synced wirelessly across a user's devices. As with the iPad, Apple will be bundling in a free Winnie the Pooh book.
5) New Enterprise features
A fifth major feature relates to enhanced support for enterprise needs:
Data Protection Security enhancements in iPhone OS4 protect email messages and attachments stored on iPhone 3GS by using the device passcode as an encryption key. New data protection APIs can be used for custom and commercial apps so that business-critical information is protected even if a device is compromised.
Mobile Device Management Deploying and managing large iPhone fleets will be even easier with iPhone OS4. New Mobile Device Management APIs can be integrated with third-party solutions to wirelessly configure and update settings, monitor compliance with corporate policies, and even wipe or lock managed iPhone devices.
Wireless App Distribution iPhone OS4 enables enterprises to securely host and wirelessly distribute in-house apps to employees over Wi-Fi and 3G. Apps can be updated without requiring users to connect to their computers.
Improved Mail & Exchange Support iPhone OS4 allows users to set up multiple Exchange ActiveSync accounts and now works with Exchange Server 2010. With the new unified inbox feature, users can see messages from all their email accounts conveniently displayed in a single inbox, or they can quickly switch between inboxes to see messages from any single account. If users receive an attachment that they want to save or edit, Mail now lets you open attachments with compatible apps from the App Store.
SSL VPN Support SSL VPN support in iPhone OS4 gives users another way to securely access enterprise resources. These new protocols can even be leveraged to connect seamlessly to a corporate network via VPN on Demand. Forthcoming apps from Juniper and Cisco will support SSL VPN on iPhone.
6) Games Center
A sixth major new feature is called Games Center. With over 50,000 gaming titles on the App Store, Apple said it will be forming a social gaming network with features to invite friends, set up matchmaking for multiplayer games, support for leaderboards, and for tracking in game achievements.
7) iAd
The seventh major feature is iAd, profiled separately. 

Other minor features highlighted in a slide of iPhone 4.0 enhancements include:

File & delete Mail search results
Search SMS/MMS messages
Web search suggestions
Choose image size in Mail messages
Bluetooth keyboards
CalDav invitations
Larger fonts for Mail, SMS & alerts
Cell data only setting
Spell check
Persistent Wi-Fi
Gift Apps
Birthday calendar
Recent Web searches
Create Playlists
Top Hit in search
5x digital zoom
Nested Playlists
CardDav
Tap to focus video
Upload workouts to Nike+
Places in Photos
iPod Out
Home screen wallpaper
Search SMS/MMS messages
Wake on wireless

A developer preview of iPhone 4 is available now, and the release will ship sometime this summer. All of the features will be available to last year's iPhone 3GS and the 3G iPod touch. The 2008 iPhone 3G and 2G iPod touch will get "many things", but will not support multitasking, as their hardware is significantly slower and has less installed RAM. iPhone OS4.0 will ship for iPad this fall.