Look at the price discrepancy between the 3G iPad's plan and an iPhone. I think if they did this, people (especially those in areas where dropped calls are frequent) would simply get a data-enabled iPod touch, with the reasoning being something along the lines of "the iPhone can't make calls here anyway, why pay an extra 40$ a month for it?" The internet everywhere is the biggest thing for the iPhone, not the phone itself. It'd cannibalize the iPhone market, and neither Apple nor AT&T want that.