apple has announced iPhone OS 4, iLounge.com has a graphical breakdown of the new features here.
we look forward to the following changes the most:
Multitasking
Multitasking is now accessible on 2009 iPhone and iPod touch models by double-tapping the Home button; you can no longer use double-clicking for the camera or other features. When you switch to the multitasking interface, the screen shifts upwards and the home screen application icons fade out to silhouettes, revealing a hidden tray of four additional icons at the bottom of the screen.
Now Mail aggregates the contents of all of your inboxes into one master inbox, letting you back out from that inbox to separate inboxes and individual mail accounts instead. Choose an individual account and you see all of the mailboxes—outgoing, inbox, drafts and any additional folders.
An optional threaded view has been added to show multiple messages that are part of the same conversation, each indicated with a double chevron arrow with the number of threaded messages alongside it.
Notes
Notes now supports wireless syncing with multiple accounts. Syncing appears to be supported for any IMAP-based Mail account, rather than being a MobileMe-specific feature.
UPDATE > john gruber writes on daringfireball…
As for why the iPhone 3G and second-generation iPod Touch don’t get multitasking with iPhone OS 4, that’s easy — those machines only have 128 MB of RAM. The 3GS and third-generation Touch both have 256.