In one of his commercials, Luke Wilson implied that the ability to talk on the phone and surf the web at the same time was unique to AT&T, even though simultaneous voice/data is really just one of the differences between GSM (AT&T, T-Mobile) and CDMA (Sprint, Verizon) network technologies.
So while we won't be able to only partially pay attention to the people we talk to on our HTC EVOs when we're in areas without 4G coverage, Sprint has confirmed in its EVO fact sheet that we will be able to half-listen to the voice on the other end of the line while we surf the web and download apps from Android Market when we're on WiMAX or WiFi. Since both are much faster than 3G, maybe it won't be as obvious to whoever you're talking to that they're not interesting enough to get your undivided attention.
Not all data use while talking on the phone is sneaky or rude, of course, but I think a lot of it is. You don't need to be able to see a person to know when he/she isn't really listening!