Private inheritance interferes with dynamic cast

Hi all. Suppose we got something like this:

class A
{
    //some code
};

class B: public SomeOtherClass, private A
{
    //some more code
};

void foo (B* b)
{
   A* a = dynamic_cast<A*> (b);
   //a is NULL!?
}

When I use this code, dynamic_cast returns NULL. However, if I change B to
be a public inheritance from A, then it works fine. So, it seems like the
kind of inheritance is affecting dynamic_cast in some way. My question is:
is this correct according the C++ standard, or is it a bug in my compiler?
I'm using a pre-release version of g++ 3.3 (I know I shouldn't be using
it,
but it's a long, non-related story).

-- 
Marc