Static function variable construction timing

What does the C++ standard say about when a static function variable is considered constructed? I have the following code (broken perhaps, but that's not the point):

    struct foo
    {
        static foo & Instance();
        foo();
        static foo * m_pInstance;
    };
    foo & foo::Instance()
    {
        static foo instance;
        return *m_pInstance;
    }
    foo::foo()
    {
        m_pInstance = this;
        foo::Instance().bar();
    } 

The source of the problem is that Instance() calls the constructor which then calls Instance() again. On two compilers (MSVC 7.1 and another), this causes no problems. The second time Instance() is called, instance is considered constructed and the code continues on. However, on another compiler (GCC 4.0), the second call to Instance() calls the constructor again. Does this fall into the category of "undefined behavior" or are one or more of these compilers broken? Just curious.