Entertainment Weekly reports that Natalie Dormer, known for the role of Anne Boleyn on The Tudors, has joined the regular cast of the second season Game of Thrones as Margaery Tyrell. Her regular cast status indicates that the role of Margaery has probably been expanded compared to George R.R. Martin's books, as she is not a major character early on in the books.
Minor spoilers follow:
Margaery Tyrell is the daughter of Lord Mace Tyrell and the sister of Loras Tyrell, the Knight of Flowers. At the beginning of the second season, she is about to be wed to Renly Baratheon (who is also her brother's lover) in order to cement his alliance with House Tyrell. With the Tyrells backing them, Renly is trying to win the Iron Throne for himself.
Source: Entertainment Weekly