My experience with VZ in the past is that when my contract ended and I bought a new phone (Palm at that time), they made me go to a higher cost plan. But if you buy the iPhone 4 while they have the unlimited data plan, they will not make you go to a more expensive plan at the end of two years unless it's then the required plan for the new phone you purchase.
For example, I currently have an expired contract and an unlimited data plan. I know if I get the iPhone 4 now, I'll be OK, but if they change to a tiered data plan before I upgrade I'll be up the creek. So I'm watching this carefully! Can't really put out the bucks for the new iPhone at the moment, but will do it anyway if it's going to cost me more in the longrun for data if I wait too long.