Those little three light plug in testers are _quite_ limited.
They simply measure voltage between the three pins of a standard receptacle. If you have voltage from the 'hot' pin to the 'neutral' pin, and voltage from the 'hot' pin to the 'ground' pin, then they light up 'correctly wired'. A bootleg ground would give exactly the same indication. In fact, if hot and ground were reversed and you had a bootleg ground, you would get the exact same 'correctly wired' reading, in a _very_ dangerous situation.
There are testers that can detect a bootleg ground; they are much more expensive. For example:
http://www.mytoolstore.com/ideal/ide05-05.html -Jon
