iPhone OS 4.0 Preview
On April 8, Apple unveiled plans for the biggest and most exciting iPhone software update yet. iPhone OS 4 will include over 100 new user features for iPhone and iPod touch owners to enjoy. And for developers, a new software development kit (SDK) offers over 1500 new APIs to create apps that are even more powerful, innovative, and amazing.
iPhone OS 4 will ship this summer for iPhone and iPod touch, and for iPad in the fall. A developer beta is available now.
The most exciting feature to me is multi-tasking ability; I remember when Apple iPad was launched, the biggest critics were not having ability to multitask. I am happy that it would not be a nag now. Some people commented that Apple should have used downgraded version of Mac OS X instead of building on iPhone OS. But now it seems that it was all well planned by Apple. 
Here is article published on builderau.com by Tom Krazit, CNET News.com
For the third year in a row, Apple used an early spring event in the US to give developers a preview of the changes coming to the iPhone OS.
Multitasking: iPhone developers and users will finally be able to switch back and forth between applications without having to shut down the app entirely to enter the new one. Probably the biggest development unveiled on Thursday, it comes with a catch: iPhone 3G users won't be able to multitask due to hardware restraints.
Folders: Apple's App Store has thousands and thousands of applications sorted in lots of categories, but once those applications made it to the iPhone they were laid out side by side in a chequerboard pattern, forcing users to scroll back and forth across multiple screens. No more: iPhone 4.0 users will be able to create folders like "Games" and drag and drop apps into those folders, simplifying the home screen.
VoIP and location: voice over internet protocol applications like Skype could be found in the App Store already, but the new OS will make it possible for them to run in the background, mimicking the way the iPhone's native dialler allows users to open up another app while remaining on the phone. And like the VoIP apps, navigation apps and others that need your current location will be able to run in the background while doing other things with the iPhone.
Inbox improvements: unified inboxes and organise-by-thread features are coming to the iPhone, which will give heavy email users some more options for organising their email.
iAd: this was perhaps the one announcement where Apple is changing the playing field: it's going to give developers and advertising agencies a way to create in-app advertisements with iAd, giving ads OS-level integration that will expand the creative possibilities. It's a clear shot at Google and its plans to transform its desktop web ad dominance into the mobile web, it's something that regular users aren't likely to be all that excited about.
References:
http://www.builderau.com.au/news/soa/Apple-takes-wraps-off-iPhone-OS-4-0/0,339028227,339302304,00.htm
http://www.apple.com/iphone/preview-iphone-os/
