No. The Iphone can always use data and talk at the same time even over 3g. It's Sprint and Verizon that cannot use data/talk together. I think it's a shortcoming of CDMA.
I believe Iphone is only over Wifi either because 3g is too slow for a decent video chat or because at&t didn't want video chats clogging up their 3g network. I believe they can use alot of data.