Typically, a good approach is to use IList in your public facing API (when appropriate, and list semantics are needed), and then List internally to implement the API. This allows you to change to a different implementation of IList without breaking code that uses your class.
The class name List may be changed in next .net framework but the interface is never going to change as interface is contract.
Note that, if your API is only going to be used in foreach loops, etc, then you might want to consider just exposing IEnumerable instead.
