Maybe you've got this already but I don't see it. The following fails
to compile (reports an ambiguity) in g++ 4.0.3:

   #include <iostream>

   template<class A, class B=int>
   class two { };

   template<class T>
   struct print {
      void operator()(void) const
         { std::cout << "1" << std::endl; }
   };

   template<template<class> class T, class A>
   struct print< T<A> > {
      void operator()(void) const
         { std::cout << "2" << std::endl; }
   };

   template<template<class, class> class T, class A, class B>
   struct print< T<A,B> > {
      void operator()(void) const
         { std::cout << "3" << std::endl; }
   };

   int main(void)
   {
      print< two<int,int> >()();
   }

Two other compilers I've tried, Portland Group and Intel, compile
the code; the executables print "3". That's what I'd have expected.

If class B isn't defaulted in class template "two," there's no
problem.

Martin