What can I do to keep my unlimited data plan on Verizon and upgrade to the iPhone 5?

Now I already know that I can buy one full price from Verizon for about $649 but I don't want to go that route, and I don't want to buy a subsidized iPhone 5 and end up losing my unlimited data plan.

Would I be able to just wait and buy a used iPhone 5 after it comes out and then have them activate the phone on my account? They wouldn't be able to take away my unlimited data would they?

You are correct. If you buy a used iphone 5 later on then yes you can still keep your unlimited data. However if you upgrade your cell phone (since your contract) is over then your unlimited plan is completely gone and you would need to choose a new data plan for the iphone 5.