Speculation is raging that Olmert is about to announce his resignation at an abruptly announced press conference. There's only one way that could be true--if he wants to annoint the next Prime Minister in exchange for some major personal benefit (like the promise of a Shimon Peres pardon). He still has the power to hand over the reigns of power the way he earned it--by becoming acting PM first and exploiting the power of incumbency.
Prediction: Olmert will hang on to the end and announce that he has decided not to run in the Kadima primary. The polls show he will lose anyway, and this way he will deflate the pressure on him to step down immediately.
Update:
Here's a clue:
Foreign Minister Tzipi Livni, who is currently in Washington, announced that she will be holding an unscheduled press conference two and a half hours after Olmert's.
Update 2: The word "unscheduled" above was an error on the part of the Jerusalem Post.