class hierarchies and smart pointers

Hi,

I have a class hierarchy whose instances need to be managed with a
smart pointer Holder<> when passed to/from lua. So I declared the
classes like this:
  class_<Base, Holder<Base> >("Base");
  class_<Derived, Holder<Base> >("Derived");

The problem is I also have functions like :
  Holder<Derived> foo()

and I really need to access the result as a Derived instance.
Luabind complains at runtime that Holder<Derived> is not registered.
I'm curious to know if anyone on the list faced this, and of course how
they solved it :)   Thanks!

- David

------------------------------------------------------------------------------
Let Crystal Reports handle the reporting - Free Crystal Reports 2008 30-Day 
trial. Simplify your report design, integration and deployment - and focus on 
what you do best, core application coding. Discover what's new with 
Crystal Reports now.  http://p.sf.net/sfu/bobj-july