Change Blindness is when someone is so focused on a visual stimuli that he/she doesn't see some other visual stimuli. This is how you don't notice billboards on the highway. Or even other cars.

During an experiment, a group of people were asked to watch a ball, on a videotape, passed around between a team. What a number of subjects didn't notice: a woman walking in the middle of the team, dressed in gorilla costume. No joke. Boing Boing explains.