Most of the time there are no geographic restrictions for car rentals when no cap or limit is placed on mileage.
But, sometimes there are, depending on the area and/or the rental company.
For example, a company may offer unlimited mileage, but only if the vehicle is driven with certain states or provinces.
While this is rare, it is always best, when making a car rental reservation, to check any links that are provided in your rate quote -- ie. terms and conditions, rental requirements, important facts for your reservation, etc. See more about Making Reservations.
Any geographic restrictions for car rentals with unlimited mileage should be listed there. If you any any doubts or questions, don't hesitate to ask a reservation agent, by calling your provider of choice's toll free number-- See Car Rental Phone Numbers