I'm pretty underwhelmed by that argument, given that my iPhone 3G on AT&T's network in NYC can barely do voice OR data, let alone both at the same time.
Edit: I also use Google Maps while talking, or check for movie times while someone is on the phone.
I too don't put much value on the simultaneous voice/data feature, but then again I have 2 phones, so I usually multitask by using two devices. Even without that though, I've never really felt a huge demand for the voice+data thing.