Phoenix V3 (from trunk) has experimental support for recursing into itself:
#include <boost/phoenix/core.hpp>
#include <boost/phoenix/scope/this.hpp>
#include <boost/phoenix/statement.hpp>
#include <boost/phoenix/operator.hpp>
#include <boost/phoenix/scope.hpp>

#include <boost/function.hpp>

#include <iostream>

int main()
{
   using boost::phoenix::_this;
   using boost::phoenix::arg_names::_1;

   boost::function<unsigned(unsigned)> factorial

       = if_else(
           _1 <= 1
         , 1
         , _1 * _this(_1 - 1)
       );
   std::cout << factorial( 3 ) << std::endl;
}

Have fun playing with it ;)
I don't accept bugfixes for this feature cause i know it is not working in every
case ... still needs a little work.
If you encounter compile errors, add some dummy phoenix expression at the end of
the recursing branch.