Near the beginning of the movie, the dialogue before the "Dammit Janet" song (when they are standing on the steps to the church), If you look above Brad and Janet's head you can see the shadow of the boom microphone. If you continue looking at this spot you will see the microphone pop into the scene at two different times before the singing starts.
I've watched it XXX (large number) of times, and I own the DVD, and it's definitely a boom mike.
In one shot it IS the light, but the mike definitely appears twice, because it moves to follow the actors!
It's not a mic. Richard O'Brian has said in many interviews that it is just a leaf hanging from the tree. And you'll notice that when it appears to follow the actors there is wind and you can see Janet's hair blowing.
The "mysterious mic" is truly the light over the church door. The pastor is everyone favorite transvestite (Tim Curry) in a different costume. If you notice, he is standing in front of "magenta and riff-raff".
Sorry, kids. It is indeed a microphone. If you watch the DVD with the commentary on, Richard O'Brien points it out and says something to the effect of "Oh, look, there's a boom microphone that's showing."
You're all right, yet all wrong. There IS a light above Janet's head, but if you look closely in the same shot, the light is on the left of the screen, then there's a shodow of the boom mic on the right. It's not a shadow of the light because it is no where near the same shape. And yes, Richard O'brian does say in the DVD commentary that there's a spot where you can see the boom's shadow. Get over it. There's a light fixture on the church and a boom.