I think he is. I think his comments were perfectly reasonable, in fact I agree strongly with them, and I even believe people should have interpreted his remarks charitably (eg, we use religion as a crutch when faced with long-term economic malaise, and it doesn't really solve anything, because it's a particularly reactionary kind of religious belief, spawned from bitterness, whatever).
But now all the people who don't want to admit (even to themselves) that they just aren't comfortable voting for a black person with a non-anglo name will find an overt reason not to do so. And others will be genuinely offended by his remarks, because they have been taught to be. A very few people will be offended by his remarks in good faith, and intelligently. Put the covert racists and scared white middle class together with those few intelligent people, and in a close election, well, it doesn't look good.
I'm admittedly an outside observer, but I "like" Obama well enough, and many people I care deeply about are not only supporting him; they are doing so with vigor and investing a lot of what's left of their faith in the system in this guy.
If it does cost him, then as much as I (or people like me) don't see a problem with what he said, I do have to ask: What the heck was he thinking? Or was his blood sugar low that day and did he let some of his (justified) bitterness about mainstream religion slip out at the wrong time? Not smart. Not smart at all. Then again, not fair, not fair at all, what's happened to him, what's gonna happen, etc.
If he loses because of this, I at least hope it will clarify for us the futility (even in the face of inevitability) of mainstream politics.