I've had 2 unlocked iphones (tmobile), 2 blackberry's, and an HTC HD2 running android over the last 4 years. Only ONCE have I ever needed to use data while on a phone call, and it was a trivial matter at best, a friend was buying a harddrive from someone and wanted to know if he was getting a good deal or not, so I pulled up newegg to see what the retail price was. Sure it was convenient, but hardly a selling point, and I would NEVER choose my carrier based on this absolutely frivolous capability.
ATT is really shooting themselves in the foot here. Most iphone idiots are barely capable of using the web in the first place, telling them they can talk while surf is just gonna be a big "zuh???" to them anyway. For the rest of us power users, we can see right through this and realize ATT is just grasping at straws, which only further drives us away.