The media enjoy decrying the preponderance of negative ads every political season, but the term itself is vague. For obvious reasons, all negative ads are about your opponent, but it seems that this is the only criterion the media use to label something as negative. In other words, if Obama talks about himself, it is a positive ad, and if he talks about McCain, it is a negative ad.
To me, I think that the more important barometer is how truthful the ad is. For example, let's say that Obama were to run an ad pointing out that McCain is Pro-Life. I wouldn't classify this as negative. Sure, if there's a deep voiceover intoning that "John McCain would send the FBI into your daughter's room late at night to make sure she has the baby" then that would be a negative ad, but only because it is untrue. Pointing out substantive policy differences between you and your opponent is not negative. But obscuring the truth or outright lying certainly is. I'd much rather see Pew put out statistics on the accuracy of the various campaigns' ads than a rating of how positive or negative they are.