I'm not sure I buy this one, though:
If Obama goes negative, he tears down his carefully cultivated "new kind of politics" facade and reveals the typical politician behind it. And if Obama is perceived as a typical politician, the central story of his campaign is decimated. Voters, particularly independents, would be disillusioned and alienated.
McCain has spent most of his career building up the same sort of not-your-typical-politician facade. Indeed, upon winning the nomination he promised both an "honorable" campaign, and to never question his opponent's patriotism. He has since broken both promises, repeatedly. (What is and isn't "honorable" in a campaign is obviously subjective, but it's hard to see the honor in this, to give one example.)
And his poll numbers have only gone up.