There is an Apple TV Advertisement that shots how in an iPhone you can run an application while you are using the phone in a call. At the end AT&T asks: Can you phone do that?

If you have an Android phone with T-Mobile, then the answer is yes. All you have to do is hit the home button while in the middle of a call. The call will be sent to the notification bar and allow the user to access any application. Since T-Mobile uses the same GSM technology as AT&T, it can handle voice and data at the same time.

Using apps during a call? My phone and my network does that! – Android and Me