It seems I have been testing this with a buggy c++ compiler or 
can't remember what was the problem previously. I now tried with gcc 
4.4.3 and this is how D and C++ differ:

---

class Foo {
  public:
  void bar() {}
};

class Bar : private Foo {
};

int main() {
  Foo *a = new Bar();
  a->bar();
}

test.cpp: In function ‘int main()’:
test.cpp:10: error: ‘Foo’ is an inaccessible base of ‘Bar’

---

module m1;

class Foo {
  public void bar() {}
}

class Bar : private foo {
}

module m2;
import m1;

void main() {
  Foo a = new Bar();
  a.bar();
}

// compiles and runs just fine
// Changing the 'Foo a = ...' into 'Bar a = ...' makes this an error