That's what the NYT is reporting:

Hillary Rodham Clinton has decided to give up her Senate seat and accept the position of secretary of state, making her the public face around the world for the administration of the man who beat her for the Democratic presidential nomination, two confidants said Friday...

Mrs. Clinton came to her decision after additional discussion with President-elect Barack Obama about the nature of her role and his plans for foreign policy, said one of the confidants, who insisted on anonymity to discuss the situation.

We'll have to wait for further confirmation.