Unlike the original 2.5G iPhone, which doesn't allow EDGE data connections while voice services are active, the iPhone 3G allows users to transfer data over the UTMS/HSDPA network while on telephone calls -- a significant enhancement.
"iPhone already gives you mobile multitasking. But 3G technology lets you multitask in more places ? without connecting via Wi-Fi. Since 3G networks enable simultaneous data and voice, you can talk on the phone while surfing the web, checking email, or using Maps. All from your 3G cellular network."