As I understand it, this is a limitation of CDMA (??). I have an HTC WinMo phone on Verizon and if I'm on a call, I can't access the internet. I can't imagine a Blackberry being any different.
That said, and since ATT is touting this "feature" or ability in rebuttal to VZW's ads, how many people on ATT know you can do this ? I'm not talking about the technical geeks either, the average "joe" and "jane". Tell someone who's trying to decide on ATT or VZW that you can do this and see what their response is.
Any Verizon phone with WIFI can do what ATT claims and at a much faster speed. I did this with my XV6800. I now have a BB Tour with no Wifi. Generally I am driving do I don't have the need to surf.