TL;DR version: Don't worry, be happy! Unlimited is here to stay for now!
Long version: I am not worried about tiered pricing, because I have a strong feeling that tiered pricing won't be introduced until the middle of this summer. I mean, think about this... we early adopters would be able to enjoy unlimited $30 data plans for the life of the contract, whether it's a one-year or two-year contract. Technically, it is possible that VZW can change the terms of the contract while you are in one, but they have to notify you somehow, and you get the chance of cancelling the contract if they change the terms. If they do that, they stand to lose money from you because then, they may not be able to collect the fees from you to pay for the retail price of the TB and then some.
Also, during a typical workweek, I am in a place that has WiFi four days out of five. While I am at a WiFi hotspot, I will be using WiFi and not have to use 3G/4G at all. I don't think I would top two or three gigs a month, if that, from 3G/4G.