From the Associated Press: "Though polls show the race essentially tied, an AP-Ipsos survey suggests that voters who can still be persuaded are more likely than committed voters to disapprove of Bush by most measures - from the economy to the war in Iraq to his handling foreign policy. Bush's advisers said uncommitted voters are easily swayed by events and thus could swing back to the president before November."
If Bush/Cheney really believes this, then it virtually guarantees a manufactured event in September or later. Might be a sudden scare (like the NY subway evacuation on "The Grid" last night) or an inexorable build up to a major event, such as the path to war in Iraq.
The fact that people are forecasting a Fall Surprise doesn't make it any less likely to happen. One of the most consistent characteristics of this administration is the willingness to follow through on plans regardless of protest. They seem to have figured out that if they power through, refuse to acknowledge any hint of ulterior motive and actually bristle at the prospect, people get confused and start doubting themselves.