From The Next Right:
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.
Ruffini is right that the Obama campaign is struggling because they're so worried about playing good defense. However, I'm not sure that Obama can pivot to offense without seriously damaging his own brand.
That would help explain why "Obama's campaign will no longer object to independent efforts that hammer John McCain". If 527's are taking care of the attacks, Barack Obama can remain in character and his storyline holds.