I found a pretty interesting post about this Pepsi Girl blog over at Clive Thompson's blog, who also writes for The Boston Globe, The New York Times Magazine and Wired, among others. Here's an excerpt.
During the Superbowl, Pepsi ran an ad called "Pop the Music", which technically was supposed to be a spot for its iTunes tie-in. But the commercial inadvertantly made a star out of a Mandy Amano, a woman who appears in the ad for about three or four seconds. Geeks swooned over her and began posting frantic mash notes on discussion boards planetwide.
Also, Brand Noise reports:
People are dedicating serious time to following the every move of the models in commercials. The two current objects of desire are "That Pepsi Girl" and "The Blonde Old Navy Girl". These sites demonstrate the new "layered" reality to advertising.