Hi -
First check to see that your mirror doesn't have a hair or speck of something sitting on it. If there is hair / dirt on your mirror, you'll see it in the viewfinder.
Next, check the upper focusing screen (the focusing screen just above the mirror) for the same thing.
If there isn't anything on the outside, then that means it's either on the insider of the focusing screen, or up on the pentaprism somewhere. Actually I've never in my life changed a focusing screen or tried to disassemble that part of a camera, so it it's really "inside", then I don't know if you can fix it yourself. I've heard it's not hard, presuming the 50d has provisions for a removable focusing screen, but I can't offer guidance. I suppose if it were me, I'd want someone who knows what they are doing to fix it up for me.
I've had specks/hairs/dust spots in the viewfinder in most of the SLRs I've ever owned. When it happened with my first camera, it bugged me a lot, but I just learned to live with it. Seems like it's a fact of life, unfortunately.
Eric
