probably not, it will become the same price per phone, or more. They will probably just charge the same, and call it a bucket.
Its just like AT$T. They just want your money, and will find a way to make your bill go up.
I am still on the $30 per month Iphone unlimited data plan, and i use an average of 8GB per month(I stream a lot of pandora, and tether at work). I have never gotten an email from them about tethering because there is no way for AT&T to differentiate between data from the phone and data from a tethered idevice if it is jailbroken. AT$T is know for taking shots in the dark, and they often go by average data usage in a month. If they see a spike, they claim tether. I will keep my unlimited untill the they wont let me anymore, and then I will be moving to sprint, because by then, sprint will have an idevice.