> Once its in the thread it seems to revert to class A's methods.

Slicing occurs here:

> thre.create_thread(*mea);

The B object is being copied as an A object. 
I think it's when the temporary boost::function0<void> object passed to create_thread() is constructed.

Try passing a refefence to A. e.g:

// warning: untested

template <typename T>
struct callable_ref
{
    T& m_ref;
    explicit callable_ref(T& ref) : m_ref(ref) {}
    void operator()() {m_ref();}
};

// no copy -> no slicing
// reference to A -> virtual B::operator() is called

thre.create_thread( callable_ref<A>(*mea) );
 
By the way, I think you can't use boost::ref, as boost::reference_wrapper does not have operator()
thre.create_thread( boost::ref(*mea) ); // doesn't work?


Maybe TR1 can do it as std::tr1::reference_wrapper should have an appropriate operator()
thre.create_thread( std::tr1::ref(*mea) ); // probably works