I was thinking about that, now if you follow the current flow, a normal 120 circuit the return voltage is on the neutral, if you bond the neutral to the ground at the receptacle, the ground terminal is usually part of the mounting strap, thus when it is installed, it is mechanically bonded also to the metal box but not only that but also the cover plate screws. Thus you (or anyone) will have "access" to the return voltage on the cover plate screws, and if physically contacted by you, you also would potentially (how much potential depends on your level of "ground" contact)become a part of or a parallel return path for that neutral under "normal operating conditions". Not just fault conditions. Doesn't sound good.
I am also thinking "GoldDigger's" statement is part of the reasoning behind the requirements to have GFCI protection when installing or changing a 2 prong ungrounded receptacle to a grounded receptacle with no ground wire. With that in mind I was wondering, if I am replacing 2 prong for 3 prong in this scenario does every receptacle need to be a gfci receptacle or just the first in circuit (like you would for "normal" gfci installation)?