As it currently stands, 4G data is still unlimited. This may change, there is no info yet.
However, if you have an unlimited 3G plan and they do implement a data cap for 4G later, if you upgrade to a 4G device later, you will more than likely have to go with a cap limit. The billing system will probably restrict you to a new plan based on the new device SIM info, etc.
My most sage advice to you is this as someone who gets their checks from a large cell carrier: if you want to be grandfathered, get the Thunderbolt. It is not a guarantee, but you'll have a better leg to stand on arguing to keep unlimited 4G while you currently have it.
I am not predicting things are going to change and nor am I suggesting that switching devices is not without its pitfalls coming up. If the billing system is hard coded to only accept certain options when switching devices, then you may find yourself without a grandfathered plan.
What I do not wish to do is have you read a load of commentary from people not intimately aware of how the systems work. Rep shopping through 611 will not let you keep certain features when they are hard coded to be locked out or unavailable.
But don't worry. For the moment at any rate, be happy