Noam, an interesting difference between Obama-Hillary and Obama-McCain is that the McCain camp hasn't, that I've seen, responded to Obama's shots with squeals about how he's betraying the "politics of hope." You'll recall that, for months, that was Howard Wolfson's first reaction every time Obama so much as looked at Hillary crosswise. As it turned out, though, Obama was mostly able to get away with some pretty rough shots at Hillary (remember his mailers about her health care mandate?) without suffering much blowback. (Or ask Bill Clinton about this, whom one suspects still mutters into the mirror about that "hit job on me" when he's brushing his teeth at night.
So, one reason McCain hasn't tried to call Obama a hypocrite may simply be that he saw it didn't work for Hillary. Another may be that McCain has always intended to run a pretty negative campaign himself. Whereas I suspect Hillary, by contrast, may have set out believing she could run a Rose Garden campaign without really engaging her opponents--and batting them down as Ken Starr clones if they came after her. That of course underestimated both Obama's talents and the party's thirst for a new face.
But on the original question, I agree that Obama can afford to--and probably needs to--unload some negativity on McCain. The common view of the race so far is that Obama is underperforming. A better view might be that McCain is overperforming--because Americans still see him primarily as a principled maverick, not a man very closely in line with his party's discredited agenda. Making that clear--judiciously, and with a more grown-up tone than the GOP has demonstrated in its attacks to date--could be what lifts Obama beyond the margin of error.
This, incidentally, might be a great place to start. Unless he's saving it for the campaign home stretch, which could be clever....