The Official Sansa Stark in the tv show Game of Thrones is Sophie Turner.
Updated June 2011: Sophie Turner is a newcomer and the role of Sansa Stark is her first on tv. She's doing well as Sansa, although, I thought they would hire someone who was much older but looked young because of certain events that happens in the book regarding Sansa.
The person I had in mind while reading the book was that chick from M2M, Marion Raven. She still looks pretty young today but I guess she's busy with her music to act in what will hopefully be a long running tv series.
Catelyn and Eddard's elder daughter, Sansa Stark is a strong believer in the world of chivalry portrayed in stories and songs. She is traditionally beautiful, with high cheekbones, vivid blue eyes, thick auburn hair like her mother and as she grows up, a tall and graceful figure. She excels at many female leisure activities, including singing, dancing, playing musical instruments, sewing, fashion, and social graces. She does not always have a good relationship with her sister Arya, whose interests are opposite in many respects and bewildering to Sansa. Her direwolf companion was named Lady.
Throughout the series, it was made obvious that she's extremely beautiful, even more so than her mother. I couldn't really think of anyone to play Sansa at the start but eventually just imagined her as the gorgeous Marion Raven, who was formerly the other half of the 90's duo M2M.