It appears, those that are grandfathered in on AT&T's unlimited data can keep it going forward with the iPhone 5. But doesn't look that way for Verizon.
http://gizmodo.com/5942731/att-iphone-5-will-keep-grandfathered-unlimited-data-plans-verizon-says-no
http://gizmodo.com/5942731/att-iphone-5-will-keep-grandfathered-unlimited-data-plans-verizon-says-no