private:
    template<class Pwd, class ExpectedPwd>
    void set_impl( Type const& value
                , typename boost::enable_if<boost::is_same<Pwd, ExpectedPwd> >::type* enable =0
                )
    {
        value_ = value;
    }

Actually ExpectedPwd is not required as template parameter, but it helps to understand the flow. It is possible to rewrite the Property as:

template<class Type, class PasswordT=void>
class Property
{

public:
    Type const& get()const
    {
        return value_;
    }



    template<class PwdT>
    void set( Type const& value)
    {
        set_impl<PwdT>(value);
    }

    void set(Type const& value)
    {
        set_impl<void>(value);
    }

private:
    template<class Pwd>
    void set_impl( Type const& value
                , typename boost::enable_if<boost::is_same<Pwd, PasswordT> >::type* enable =0
                )
    {
        value_ = value;
    }

private:
    Type value_;
};