Pundit musings about any convention bounces are completely nonsensical this year. The schedule and calendar of the 2008 presidential campaign has no modern parallel. In turn, looking at past convention impacts on polling won't provide accurate historical context.
Traditionally, conventions are in the heart of summer, before the general election campaign has been fully joined. Nominees usually get four days of puff press that serves to introduce themselves to a wide portion of the electorate, which temporarily gooses poll numbers to varying degrees.
This year is completely different. With back-to-back conventions scheduled at the very end of summer, the campaigns had little incentive to wait before attacking. Obama and McCain have been jabbing at each other all summer. Negative attack lines towards both are already well established (though will surely be honed further.)
Four days of puff press can't detox what has already been injected into the bloodstream. And puff press shouldn't even be expected, since rival campaigns have dropped the traditional courtesy of holding fire during conventions.
Futhermore, since the conventions are back-to-back, there won't even be a window for professionally-executed polling to occur following Obama's acceptance speech (though expect some shoddy quickie polls). We'll only get some remotely accurate polling after both conventions are finished.
What I do expect, following a very mild tightening in the polls in recent days, is a return to what we've generally seen all summer (and what we see in today's ABC/W. Post poll): Obama in the upper 40s, McCain in the lower 40s. That merely reflects where the nation is ideologically right now.
If we start seeing Obama consistently break 50 after both conventions, that would be a better-than-expected success, an indication that Obama was assuring undecided voters he can deliver on the change the majority of country wants to see.
If we start seeing McCain consistently break 45 after both conventions, that would indicate McCain was having success sowing doubt about Obama among undecideds, which would likely make this a very close race to the finish.
But any poll movement should not be described as a "bounce" following a one-sided presentation. After back-to-back conventions, poll movement would reflect the impact on the electorate following the first major clash of the campaign.