Nope, she will announce with Bob and Mark on their radio show September 2, 2011 (Friday news dump).
Onyx, you know I love you, but on this, we disagree.
The big Moment will be squandered if Sarah just 'phones it in' to the Mark and Bob radio show. It's got to happen at her Indianola speech. They even had to move the venue to accommodate the larger than expected crowd that's coming. Press from all over the world is going to be there, and the place will be packed from wall to wall in anticipation.
Her speech would be anti-climactic if she spilled the beans a day beforehand. I really think that she's going to figure out some way to tell them first, without giving the show away.