Why was std::hash not defined for std::weak_ptr in C++0x?

After reading the discussion on operator< for std::weak_ptr, I can't see any reason why defining std::hash to use the control block for std::weak_ptr wouldn't work. I also can't believe that this was ignored by the standards committee. Has anybody read the relevant discussion on the topic?

Edit: Discussion on operator< for std::weak_ptr http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1590.html