If we are only grandfathered for 2 years, then that is even more of a reason to keep it as long as we can.
Everyone keeps thinking that their data usage on the 3G network will be the same as it is now, with the new iPhone and apps. But the first time you watch a Netflix movie over the 3G because you don't have access to WiFi, then you might wish you had the unlimited data plan.
New phone and new apps mean greater downloads. AT&T isn't saving you any money, they are counting on data overage charges.
Now had the 2GB plan cost $15 per month, that would have gotten my attention.