Nothing, unlike if you chew wintergreen lifesavers in a dark room in front of a mirror. They at least send sparks around your teeth.

The girl who is waiting will be gone when the lights go on.:)

Moves you one step closer to the nuthouse! (just kidding)

Yes.
It designates the speaker as a fool. :o)

man!!..its true ..a group of us back in the early 70's saw her ....but we just don't know if it was before or after the 'acid' we dropped :):)

I am not sure about Bloody Mary but I do know that [similar to what MikeD81 has said regarding mirrors] people have sat in a enclosed area with a mirror surrounded by black curtains and the idea is that your dear departed relative(s) will eventually appear - now I attempted to try it one night for curiousity sake but I found it unnerving staring into darkness so I quickly gave up. The reference to ghosts and mirrors is seen in the movies The Grudge and if you'll notice, that supernatural events happened with a mirror close by in the movie "The Shining." But if you want to read about a real evil occurrence read up on the Hungarian "Blood Countess" Elizabeth Bathory! She was a noble that grew up immersed in a violent time with bloodthirsty rulers all around but she was personally responsible for the murders of many young women. Wikipedia has a great article on her. Anyways, sorry I sort of diverged there!

You will still be standing in the dark in your bathroom:)

Haha! No!
I tried it when I was little and was terrified that I was next on Mary's list.

It does'nt do anything but make your wife say:Who is this Mary person?

I did it,me and my friend were on the phone and we called her at the same time."Bloody Mary,Bloody Mary,Bloody Mary!!!"We each saw a tad of our reflection with red tint on the mirror.Mmmm.Bloody Mary hates me!
But I have been known to have nervous break-downs when I was a kid.

Any way,some links:
http://www.snopes.com/horrors/ghosts/bloodymary.asp
http://www.halloween-website.com/bloody_mary_stories.htm