Preschoolers who often watch commercials usually believe them. They also tend to believe social stereotypes and try to persuade adults to give them the things advertised. The study also said those kids may get desensitized to violence, become less verbal and have less social communication, but I think those things may be a result of watching TV, movies, etc. in general and not just from the commercials.
Either way, it's probably a good idea not to let young children get a lot of media exposure.