Maybe. If you deside to go with a 4g plan. It depends on how the providers deside to offer their data plans. They could allow you to keep your unlimited 3G data plan and charge extra for the 4g or. One plan for all which would probably eliminate the unlimited.
I kind of think AT&T is trying to kill off unlimited data plans anyway, aren't they? (Or at least until another carrier offers it on the iPhone..c'mon competition, c'mon free enterprise....)