Covariant return types doesn't work (with g++ 4.1.2)

Hi,

I have been totally confused about the covariant return type feature
in C++. Below is an example code I wrote that doesn't compile with g++
4.1.2 (on Fedora 8)

class virt_base
{
public:
    virt_base()
    {
    }

    virtual ~virt_base()
    {

    }

    virtual virt_base* cut()
    {
        return new virt_base();
    }

    void say_hi()
    {
        cout <<"hi!!! " <<endl;
    }
};

class virt_derived
: public virt_base
{
public:
    virt_derived()
    {
    }

    ~virt_derived()
    {

    }

    virtual virt_derived* cut()
    {
        return new virt_derived();
    }

    void say_hi()
    {
        cout <<"HI!!!! " <<endl;
    }
};

int main()
{
    virt_base* my_virt_derived = new virt_derived();
    virt_derived* new_virt_derived = my_virt_derived->cut(); // g++
complains here: invalid
                                                                                        //
conversion from 'virt_base*' to
                                                                                        //
'virt_derived*'
    new_virt_derived->say_hi();
}

------------
It is very weird. Can somebody enlighten me what went wrong here?
Thanks in advance.