I was browsing the mongodb code in doxygen (I'm thinking of "stealing" your ptr<T> class) and noticed that the operator->() and operator*() (dereference) are not const correct:

T* operator->() const

{ return _p; }
T& operator*() const { return *_p; }
operator T* () const { return _p; }

should be:

const T* operator->() const

{ return _p; }
const T& operator*() const { return *_p; }
operator const T* () const { return _p; }

// ... and if you want non-const versions.
T* operator->()

{ return _p; }
T& operator*() { return *_p; }
operator T* () { return _p; }

If there is some rationale for violating the const correctness, it should at least be documented in a comment (didn't notice anything in the doxygen documentation regarding this).