The main reason this particular ad is dumb is because it's so painfully juvenile: you called us a celebrity, so we're going to call you a celebrity! Nyah nyah nyah. More generally, this kind of attack (McCain is a celebrity too! McCain is the real elitist! McCain wears $500 shoes!) fails because everyone knows that liberals aren't serious about it. We're just being cute.
Attack ads only work when (a) they have a grain of truth to get them started and (b) they're seen as attacks that genuinely represent what the attackers think — even if what they think is stupid, unfair, and revolting. McCain's attacks have been all of these things, but at the same time it's entirely plausible that conservatives really do think Obama is an empty suit, really do think he's a garden variety liberal tax raiser, and really do think he's a dangerous Chamberlain-esque appeaser. Conversely, nobody believes that liberals really care about McCain's shoes or his appearances on Saturday Night Live.
What makes this especially frustrating is that if the Obama campaign wants to go on the attack, there are plenty of sincere ways to do it. McCain really did abandon his famous moderation once he decided that support from the conservative base was the only way to win the Republican nomination. McCain really has run up a staggering number of politically motivated flip-flops. McCain really has covered himself in mud by accusing Obama of not caring about American security. McCain really is Bush on steroids when it comes to American military involvement abroad. And McCain really is 71 years old and stumbles a lot. If you want to take a cheap shot at him a la the Britney/Paris ad, then put one together that demonstrates his increasing "confusion" about foreign affairs. And don't tell me you can't do it subtly enough that it can't be defended in exactly the same way that McCain's partisans defended the Britney ad.
Or, don't do any of this stuff. Run a positive, issues-based campaign instead. But if you are going to attack, for God's sake do it right. Nobody's going to buy the celebrity/elitist/Ferragamo shoes nonsense that the liberal blogosphere has been pushing except as evidence that we're in our dotage too. It's time to pick up our game a notch.