I feel liek someone here would know. I tried googling, but couldnt find anything. It seems to deal with the viewfinder. It came out of the front of the camera body as I was trying to switch lenses.
It's your focusing screen. You can change them out depending on what you want.
I have an Ef-G focusing screen for my 40D, it has grid lines which makes it easier to line up your image correctly. You can change them out if you'd like.
