I knew it was too good to be true.
According to the local Verizon Wireless rep I talked to on the phone, if I ever upgrade to a newer iPhone my current contract with my new iPhone 4 is invalidated, and - surprise - I would have to create a brand-new contract (i.e., goodbye unlmited data plan). It's not a problem now, but down the road, when the iPhone 5 or 6 or 7 comes out, sure, I can keep my current contract month-to-month with unlimited data. But by then my iPhone 4 will be an antique.
Maybe in the future unlimited data plans will be commonplace, like long-distance calling is now (compared to 10 years ago when you had toll charges). But for some reason, I doubt it. It's the cell carriers golden goose. They're never going to have unlimited data plans again, especially considering how much people communicate and do business on the Internet now.