Where I work, if I don't use a certain percentage of my earned PTO by Dec 31, it just goes away. They don't cut me a check for it.
I think I'd get paid for unused PTO if I quit or was fired, but I couldn't accumulate more than something like 5 weeks worth.
I'd love to work for a company that had both unlimited and required PTO. If you don't take enough days off for say six months, they force you to take a week or two off.