I imagine it would depend on the state. In California, I believe they can get away with putting "0" as the number of vacation days you are entitled to, so any of the "unlimited" days you take off are really at the whim of your employer.
Where I am in Canada (and I imagine many states) there are legal minimum requirements so I officially get "3 weeks" vacation, but take a couple weeks on top of that using the "unlimited" policy. So "unused" vacation would be paid out according to the official 3 weeks policy in my case. I could see something similar being possible in Europe, but I think in practice europeans already take more paid time off than most North American's with unlimited vacation.
Where I am in Canada (and I imagine many states) there are legal minimum requirements so I officially get "3 weeks" vacation, but take a couple weeks on top of that using the "unlimited" policy. So "unused" vacation would be paid out according to the official 3 weeks policy in my case. I could see something similar being possible in Europe, but I think in practice europeans already take more paid time off than most North American's with unlimited vacation.