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There are not technical reasons, only Apple marketing policies, but the iPhone is not allowed to run multiple applications simultaneously , by design only the phone, mail, ipod, and maybe some other Apple application can run in the backgroud.

This is of course not the desired behavior for many cases…

Multitasking you can, for example:

• Have a messaging service available at any time.
• Have an IP phone (e.g. skype, gizmo5) available all time.
• Pause a game,  go to write an email, read the news, and go back to the game…etc.
• etc

So this is what’s needed to enable Multitasking in your iPhone:

1. In order to enable it, the iPhone must be jailbroken, if  it’s not, there’s a jailbreak how to here.
2. From Cydia (or Icy), search and install the backgrounder app. This will provide the availability to run applications in the background.
3. From Cydia (or Icy), search and install the application kirikae. This will provide a task manager that will let you switch between open applications and close them. You can also add favorite apps to open faster.
4. Have Fun