It doesn't seem to work with default arguments. :-(
Here is my test case (under gcc 4.7.2 -std=c++0x):
#include "boost/type_traits/has_call.hpp"
#include <cassert>
template<typename Sig, typename F>
bool HasCall(F const&)
{ return boost::has_call<F, Sig>::value; }
void DefaultVal(int = 0) {}
int main()
{
assert(HasCall<void()>(DefaultVal));
}