You may not have been gradfathered in, but the 20 bucks is a grandfathered option, that they offered you. PS...there is also an unlimited 5 dollar data plan.
PS...I kinda laughed about the battery thing, if t-mobile gives out free batteries to everyone they will go out of business. Let us know if it works though.
Those battery's dont cost them jack but charge you $49 for it in their store. It isnt going to break them. And i was using the $5 data plan on my iPhone.