The accepted answer states "it's not permitted to have a 3 prong receptacle without the ground wire being attached." This is incorrect.
406.4(d)2(b)
A non-grounding type receptacle(s) shall be permitted to be replaced with a ground fault circuit interrupter type of receptacle(s). These receptacles shall be marked "No Equipment Ground". An Equipment grounding conductor shall not be connected from the ground fault circuit interrupter type receptacle to any outlet supplied from the ground fault circuit interrupter receptacle
406.4(D)(2)(C)
A non-grounding type receptacle(s) shall be permitted to be replaced with a grounding type receptacle(s) where supplied through a ground-fault circuit interrupter. Grounding-type receptacles supplied through the ground fault circuit interrupter shall be marked "GFCI Protected" and "No Equipment Ground". An equipment grounding conductor shall not be connected between the grounding type receptacles.
Thus, having a three-prong recepticle with no ground is allowed, as long as it's GFCI-protected (either by being a GFCI, or being on the load-side of a GFCI) and has a "No Equipment Ground" sticker.
