Comment the interface, implementation or both?

I imagine that we all (when we can be bothered!) comment our interfaces. e.g.

/// <summary>
/// Foo Interface
/// </summary>
public interface Foo
{
    /// <summary>
    /// Will 'bar'
    /// </summary>
    /// <param name="wibble">Wibble factor</param>
    void Bar(string wibble);
}

Do you also comment the implementation (which may also be provided to clients, e.g. as part of a a library)? If so how do you manage keeping the two in sync? Or do you just add a 'See interface for documentation' comment?