If you are referring to the commercial with the setting of a girl running in the outdoors that is Deb D'Agostino. Deb is a model who resides in New York City.
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It is French fashion model Alexa de Puivert.Stunning, isn't she?
There are quite a few Stelara commercials and many brunettes, so, you'll have to specify, which commercial (airdate; your locale; etc) and brunette (time frame). If you find the ad on the web, give us the link (either as an edit to the question or as an "answer"), and indicate the time point at which the brunette appears.
The brunette actress in the Cingular commercials is Milana Vayntrub. She began her acting career in 1995 appearing in the television show, ER.
The actress is an amazing Brazilian model named Carolina Brandão. You won't find many images of her, but if you search her name, you will get a couple of decent hits.
Her name is Alice. I don't remember her last name.