Looking at mine, I think it is meant to be there, although I have never really noticed it before.
Does yours reflect light when you're looking through the viewfinder though?
I have just emailed the seller (a camera shop in Cardiff), this is what I wrote.
I recieved the Camera (Sony A200) on Wednesday. It works well but I have noticed what seems to be a fault in the viewfinder. Whenever I look into the viewfinder, there is something which reflects any light getting into the back of the viewfinder into my eye which causes an extremely annoying 'artifact' on the left hand side of the viewfinder (only the left). The only way to stop this is to completely cover the area around the viewfinder with my hand to stop the light getting in which is impractical in normal circumstances. I think I know what is causing the reflections (some metal rings within the viewfinder) but they seem to be pretty normal and seem even on both sides, so I really don't know why this is happening. When I look at the viewfinder from a distance it looks pretty normal, the external plastic curves at the edges but I think it's supposed to be like that which makes it even more confusing. I have no idea what the cause of the problem is. I hope you can help!
Thanks
I would have written more but there was a character limit.
Hopefully I'll get a reply back soon.
Just to clarify, before I'd noticed this problem, I had not touched any of the delicate parts of the camera at all. When I recieved it, I got it out of it's box, put the lens on, done. Didn't touch anything like the sensor or the external plastic on the viewfinder at all. So I don't think it's something that's been manually dislodged..
What I really want to know now is exactly what's wrong... Or if it's supposed to be like that

I think that's what's aggravating me the most, simply not knowing what the hell is actually wrong. I've been googling various things pretty much all day and have found no results at all which is a bit disconscerting. :shrug: