The key word here is multitasking, a major concept in modern operating systems. It is the ability of the operating system that allows it to run more than one application at the same time, something quite common with our computers, but not possible so far with mobile phones including the iPhone. Apple has built iPhone OS 4.0, the new operating system for the iPhone to be a multitasking OS, and VoIP applications are favored, because they will be allowed to run in the background.

Why VoIP is favored hasn’t been explicitly stated, but the obvious reasons are the importance of VoIP as a key technology and hence money earner, and the fact that communication applications actually need to run alongside other applications because they achieve their purpose best when idle (rather than being off) ready to spring alive on any communication event, like a call.

Source: About.com