I totally agree with Jason on this. I might have sent a shot across McCain's bow on Keating--a campaign operative offhandedly connecting Keating to McCain's views on deregulation or something. And it was probably helpful to remind team McCain of the risks of their new mud-wrestling strategy.
But I wouldn't have dusted off the medieval weaponry just yet. Given the polls and voters' economic preoccupations, McCain's attacks mostly looked desperate and irrelevant. They were more likely to hurt him than Obama. Now, as Jason says, it's a little harder for Obama to claim he's the one focused on what voters truly care about.
Also, in campaign narrative terms, Obama would have been better off letting McCain spend a few more days throwing punches, then counterpunching once the media had pinned the negativity on McCain. Obama's been a great rope-a-doper this last year-and-a-half. I'm a little surprised he got away from that here.
The only caveat is that, as I say, Keating is arguably pretty relevant to the current debate, since it was ultimately a matter of banking regulation and what happens when the regulators don't do their jobs (and when compromised politicians don't let them). I haven't been able to watch the entire Keating video (firewall issues), but my understanding is that the second half hits the policy implications pretty hard.