When I had my ultrasound, the girl was fine, but the boy had a choroid plexus cyst which is in the brain, and occurs in up to 4% in babies. We did research on the net and found out the same thing as you, that it can be a marker for downs syndrome, but as all his fetal measurements came back NAD, my OB isn't worried. If your OB had had any concerns at all, he would have sent you off for a 'target scan', which is a much more detailed scan of the area of concern.
As he hasn't, I would take his advice and not worry as it sounds like it is very unlikely that anything is wrong with your baby.