It's a little hard to explain why, as I haven't read it in quite some time, but "Watership Down" was the first thing that popped into my mind when I heard about McCain's house gaffe.  As those of you that have read it may recall, the rabbits in the book can't count beyond the number four and every number higher simply because "hrair" meaning a thousand but really just referring to any big number.  I think this can be used, in a way, on both sides of this. 

Pointing out that McCain can't count high enough to know how many houses he owns, as Kaine did, is all well and good, but I think the better tactic is to play along with it.  The reason McCain can't recall is that the specific number doesn't really matter; it's simply a lot.  Having three houses would be ridiculous, four would be absurd, but more than that?  What does it matter the specific number if it's more than four?  It's hrair, and that's too many.