Don't be silly. She decided to run over two months ago.
Announcing now would be tactically unwise.
Contrary to the views above, she is not at risk to having people leave to support others.
Non-candidate that she is, she has the best organized most effective campaign organization in Iowa and when she does get in, she should be the favorite to win the Republican side in Iowa immediately.
She also has people working New Hampshire. I know less about what is going on there but you would bet that she is looking at New Hampshire as the opportunity to put Romney away; and South Carolina as the opportunity to the inside track for the nomination.