1) Yes, but as you note it's trivial to add if it's important. I had that in my own implementation of intrusive_ptr (before those were in Boost) but dropped it because I never used it. Not such a big deal on Win32, but I remember it being painful on Sun / Sparc systems (am I dating myself?).
boost::shared_ptr have reference counting for various platforms, and there's no policy to have custom reference counter, so, although I've never coded for Sun / Sparc systems, it seems possible to do what boost::shared_ptr does.
3) A design feature for me. It makes copy on write semantics easier to implement. You might consider putting in "bool is_shared() const { return m_reference_counter > 1; }" for that case.
I would rather put "bool is_shared() const { return m_reference_counter > 1; }" as protected member, and m_reference_counter as private one. Do you need it for something like "intrusive_from_this()" form Boost.Statechart ?