Update: In response to the rather vocal outcries of many of its subscribers on the web, Verizon has clarified what will happen to 3G/4G data plans explicitly. The takeaway is this: anyone purchasing a smartphone from this summer forward on subsidy pricing will be pushed into tiered/shared data. If you choose not to buy a smartphone on subsidy, you can keep your unlimited plan if you choose to.
This means if you renew your 2-year agreement, from this summer forward, on any line by buying a "discounted" phone, you lose unlimited.
Here are the exact statements: