AT&T Unlimited plans are already gone with 3G unless you are grandfathered. And those that are grandfathered will eventually have to give it up anyway. If you want tethering, you have to give it up. If you want to go to LTE, you'll have to give it up.
My hunch is that when Verizon announces their iPhone data rates/plans, the unlimited option will either be gone or cost more than their current unlimited plan, which is $30/mo. And even so, it probably won't last for long even if they do keep it, given the strain all of these iPhones is going to put on their network.
Unfortunately, the days of unlimited data over wireless is gone. And the complete failure of Net Neutrality means there will be no remedy for the foreseeable future.