It's fairly bad. He's been trying (to some extent) to paint Obama as the elitist, and also claiming that the economy is just fine, when the guy is so loaded he can't even tell you how many houses he has. Makes him look out of touch in general, and it's especially bad considering the housing market at the moment.
In fairness to him (not that he needs much defense around here, obviously), once you get to a certain level of wealth this is not as simple question is it might seem. For instance, he owns two condos that are attached...is that one or two? Do you count the residence for his daughter, which is probably in his name? Do you count rental properties? I think the question asked how many "homes," not "houses," which makes it even more vague. Depending what you count, and how, the number can actually range anywhere from 4 to like 11 or 12.
At the same time, this doesn't help the perception it's going to cause among the majority of people who can barely afford one house, or who don't own one at all.
Also kind of sad is the fact that this proved that there was absolutely nothing which the McCain campaign would not try to tie his POW status into. It's turning into his version of Rudy's 9/11, where it's pretty much all they say to anything. The only thing that I think has saved it so far from becoming a running joke is that it's generally not McCain himself saying it.
Now, all of that sounds pretty anti-McCain, but I'll say that personally I think the entire thing should have been a non-issue, and the question itself was a loaded one to which there was no "right" answer. Much like Obama's middle name, it's the kind of silly crap that makes our whole process seem absurd sometimes. But if you're asking about the actual political consequences of it, I'd say he took a pretty decent hit today. And it looks like this is going to set off a flurry of back-and-forth negative campaigning as well, so we'll have to wait a bit to see who comes out on top.