In PA, ATV's are registered thru the PA-DCNR office, and, yes, you do get a license plate. (Does that make it a "license"?)  If you use them on public property, they need to be registered, and have show proof of liability insurance.  Farmers do not need to register them to use them on their own property.

I think Boog was asking about IN, and I do think he needs to get a response about what is pertinent to IN, as I suspect the answer will vary from state to state.