Maybe you could rig up a cheap web cam to look through the peep hole in the door. This might not work if your computer is all the way on the other side of the room, but it's an idea.
I have a web cam set up on my desk in my dorm room that points towards the door. I have it turned on when i leave the room to go to the restroom or just any general time so I can see who is at the door, since I face away from the door while sitting at my desk. It is motion sensitive and it even has a setting so only motion in a certain portion of the display is detected. It either does snap shots or video and can be saved on the hard drive or emailed to an email address.
EDIT: I just thought of this and it's kinda funny. You could remove the peep hole once installed and place some kinda of tube in the hole of similar size. You could then run the tube to where ever you sit and at the end of the tube have one of the "squeezy rubber ball thingies" (I can't remember what it's called for some reason but hopefully you are following me). You could then fill the ball and 1/2 the tube up with some kinda of non-toxic ink and whenever you hear the door knock you could squeeze the ball, forcing the ink through and out the tube, and cover the individual(s) outside the door in ink (hoping that it's not really one of your friends coming over to visit). Then you could easily identify the pranksters.
