Dutch researchers found that when women viewed a female celebrity wearing a certain shoe, they experienced heightened activity in the part of the brain associated with affection. This brain activity made it more likely that they would associate the celebrity with the shoe. Similar activity did not occur when a woman saw a picture of a similarly attractive non-celeb. People we "know" influence us and only the pervasiveness of the celebrity's fame (aka how well we know them), increased the brain activity. Conversely, attractiveness or perceived expertise about the product had no impact.
This study could explain why many brands have stopped using unknown models, instead having prominent actresses pitch mascara or shampoo to women, as well as the motivation behind the abundance of product placements in the Sex and the City movies. Now they just have to make sure we can afford those shoes the celebrities are wearing.