As you may know that apple intends to launch iPhone OS 4.0 and iPhone 4G at Worldwide Developers Conference 2010 but before its launch, according to Apple resources they have decided to include multitasking to iPhone OS 4.0.
All iPhone customers know about this problem that their device is not completely capable to run different applications whereas the jail-broken iPhone can perform these actions. I think iPhone OS has the hidden features for multitasking which Apple has restricted due to some security issues. But now Apple has decided to include multitasking in iphone OS 4.0 which will enable the most demanding iPhone with multitasking to make this possible for users to run different apps simultaneously.
The second reason to include multitasking to iPhone OS 4.0 is that the rival webOS supports multitasking feature to hold its position in top of mobile market, Apple has decided to include this feature to the next generation of iPhone.