If I'm John McCain I'm certainly regretting having said that about all my houses. On the other hand, in politics memes tend to stick when they reinforce things that people already intuitively believe. Hence George HW Bush in the supermarket was a disaster because everyone already had the sense that Bush was a disconnected upper-class twit. I don't think many people feel that way about McCain, and even if they're exposed to lots of information about his wealth, he just doesn't come across as a pampered rich guy. By the same token McCain's current counterattack--to go after Obama's house and its connection to Tony Rezko--seems likely to be as ineffective as it was when Hillary tried it during the primaries. Obama doesnt seem the least bit shady or ethically compromised.
In a pure Machievellian sense I'd say the best ad hominem attacks for each side, respectively, are to paint McCain as culturally out of touch in an elderly, doddering way, and Obama as an arrogant elitist. But imperious millionaire and corrupt hack are just not the narratives I see sticking.
Update: A friend emails arguing that Obama's new attack ad about McCain's homes "is clearly meant to raise the age issue." On second viewing, I'd say it does arguably press the senility button: "Asked how many houses he has, McCain lost track. He couldn't remember." That's rather more loaded than just saying, "McCain wasn't sure."