No, they only ask that you have their one of their smartphone data plans (even grandfathered unlimited data). At least until they roll out their LTE network. There are no additional fees for HSPA+. However, I would very much expect them to change it and charge extra for LTE when they start rolling it out starting in the summer. Especially considering LTE will probably require a more advanced device to use it, and those type of devices will start to appear around the same time.