Technical specifications can't just "change" on a whim. EDGE/GPRS uses the GSM radio to download data thus negating ANY possibility of being on a phone call (using the GSM radio) while simultaneously loading data. Obviously this would kill the idea of an iTunes radio on EDGE.
Now of course you can receive a phone call while you have Safari OPEN and NOT loading data.
Thats one of the benefits of '3G' UMTS/HSDPA in that you have separate channels for voice and data and both can be used simultaneously.
Even with a 3G iPhone, I can't Imagine AT&T would be encouraging constant streaming of radio on the iPhone. That part baffles me.
Just because you can't download data while making a call doesn't mean that the phone can't pause the download when you receive a call. I don't care if my web page continues to download while I'm on the phone, as long as the call doesn't go straight to voicemail.