Generally presidential candidates get a bounce when they announce their running mate, and another bounce when they have their convention. These typically fade away before Election Day. Therefore, I suspect that the end of August / beginning of September is probably the high-water mark for the Romney campaign. If Obama still has any kind of lead at all following the Republican convention, I think he'll be overwhelmingly likely to win. Basically I see the election in more or less the same way I've seen it for months now: Obama is a solid favorite to win reelection, but by no means a certainty. As an Obama supporter, I don't think there is any reason to worry unless Obama is actually behind in the polls by the last week of September.
As for undecideds breaking against the incumbent...Nate Silver at FiveThirtyEight has done a statistical analysis of this, and found that it's largely a myth. Undecideds are not much more likely to vote for the challenger than the incumbent in presidential elections. In any case, there are an unusually low number of undecided voters in this election.