I've heard reports that one person is dead and 49 are injured (though the newspaper reports say 79 wounded). I talked to tourists whose hotels were around the square and they reported hearing gun shots for a couple hours during the night and many sirons of ambulances. Here is an article in the Egyptian Gazette describing the event.
I've heard that the military arrived at Tahrir Square in large numbers around 2-3am. I was in the square earlier that night, around 6pm, and everything was peaceful.
Here is an AlJazeera news report with videos of the events:
This video is also available at this link: http://english.aljazeera.net/video/ With the heading, "Clashes Erupt Around Cairo's Tahrir Square"
In other news, the verdict for the blogger put on trial for comments made about the military on his blog has been postponed. Here is an article about this case: