Re: std::vector anomally

I have stumbled across an interesting anomally (well... I find it
interesting). In VS8 the following gives a compiler error that method
Xxx::readOnly is attempting to return a reference to a local value (see
"problem here" below).

class Xxx{
private:
  std::vector< bool >* _readOnly;
public:
  Xxx();
  virtual ~Xxx();
  const bool& readOnly( const unsigned int& i_ ) const;
};

Xxx::Xxx(){ _readOnly = new std::vector< bool >; }
Xxx::~Xxx(){ delete _readOnly; };

const bool&
Xxx::readOnly( const unsigned int& i_ ) const{
  return _readOnly->at( i_ ); // problem here.
};

If I change the three occurances of bool to double or std::string the
problem disappears. When I move it to VS6 the problem with bool does not
occur. Is there something about bool that would cause std::vector::at to not
return a referenced? Is there some project and/or solution setting that I
may have changed that would cause this anomally with bool?