If you have unlimited data right now on a smartphone using the same phone#, you are grandfathered in regardless if you are month to month or starting a new 2 year contract. As long as you already had unlimited data you retain it period. You can switch plans, upgrade phone, change features, etc... as long as you are active on Verizons network and don't disconnect or change accounts.
This is probably the most concise way to put it. If you currently have unlimited data and a Verizon smartphone, you will continue to get unlimited data until they make it a policy change. It does not matter whether you're month to month or under contract. Tiered only applies to new smartphone lines.
All existing customers currently with unlimited data will get unlimited data on their next smartphone unless Verizon changes their policy. I wouldn't expect them to change their policy
UNLESS AT&T changes their policy regarding grandfathering of customers. If one of their reps try to convince you that you have to go on tiered plans, do not listen. Also, if you're interested in tethering, you can get unlimited tethering for $30 dollars on top of the $30 unlimited plan. They will try to tell you that you have to go with the 2 GB plan, but keep pestering them until they give you the unlimited...
We will probably see it end sometime in the future, but hopefully not for a while.