The Clinton campaign just sent out this statement:
While Senator Clinton has made clear throughout this process that she will do whatever she can to elect a Democrat to the White House, she is not seeking the vice presidency, and no one speaks for her but her. The choice here is Senator Obama's and his alone.
My guess is her campaign scanned the reaction to her Tuesday night speech and to the news she had told New York lawmakers she would be open to running for vice-president, and realised they had gone too far. Lest they be seen as attempting to strong-arm Obama into choosing her, they pulled back.
For anyone not adept at parsing US political speech: This doesn't mean Clinton doesn't want the vice-presidency, it's merely a signal she will lay off Obama and won't make an all-out push to join the ticket.