It's legal, but it ain't moral.

Just found a great C++ FAQ site which answered the one interview question I couldn't. And the best thing is that the technique described is not recommended so I don't mind not being aware of it:

You know you can override member functions in C++ without them being declared virtual; and you know when and why you should declare them virtual.

"Experienced C++ programmers will sometimes redefine a non-virtual function for efficiency (e.g., if the derived class implementation can make better use of the derived class's resources)"

b.