This is mostly speculation, but it does have some historical evidence to support it:
-It seems that the tiered data plans will be for 4g, not for 3g data. This means that as long as you don't get a 4g device, you should be able to keep your unlimited plan. Like when they went from America's choice to Nationwide data pricing. Older phones used to use plan minutes to use data. As the phones improved (and thus so did their capability to consume data) They switched to the nationwide pricing. Higher end phones required nationwide plans to activate, but if you kept with one of the more basic options, you could continue using plan minutes to surf the web.
-If they do change 3g, they'll grandfather current plans for current users/devices at least until you are up to renew your contract. (like they are doing with the 3g multimedia devices). But if you change your phone at all, you might lose your plan. (or you might be ok keeping yuor plan as long as you don't get a 4g capable device)
At the very least, we know that tiered data is coming with 4g and you won't be able to move your current 3g plan and apply it to 4g data.